Screen Printing/Making the Stencil/Photo Emulsion
How To 
One of the often preferred methods for making silk screening stencils is the photo emulsion technique. Photo emulsion is thick liquid substance which reacts to light. Essentially, photo emulsion becomes "tougher" when exposed to light, making it more difficult to remove from surfaces. That's perfect for creating a stencil: we'll "toughen" up parts of the stencil that we want to keep solid, and then wash away the rest.
Applying the Emulsion 
The way we do this is to spread a thin layer of the emulsion over both sides of our screen. You can do this with either a "scoop coater" or a standard squeegee.
To clarify in this section, the "top" of the screen is the side that sits embedded in the wooden frame, and is the side from which you apply screen-printing ink. The "bottom" is the side where the screen is flush with the wood frame, and is placed side-down when printing.
Since you'll be applying the emulsion to both sides, you will need some temporary way of supporting the frame when it is side-down. Thumb tacks stuck into the corners of your frame work well for this. You could avoid this if you did the top first, and then the bottom, since the frame itself holds the top up off the surface, but you'll also want to dry the screen with the bottom side down, so you'll need to support it then, anyway.
Lay the screen flat, right side up, and pour a thin line of emulsion along one edge of the top side of your screen. Use smooth, even, and fairly light strokes of the squeegee to apply a smooth and even layer. Once the top side is coated, flip it over and repeat on the bottom. You'll want to make sure, therefore, that your squeegee fits inside the frame on the bottom side. It is best if it will fit going both the long way and the short way, but as long as it at least fits the long way, you should be ok. Note that after doing the top side, you'll probably have quite a bit of emulsion already pushed through the screen on the bottom side. That's fine, but you'll probably want to add more to the bottom side, still, to get a good coat.
Make sure to cover entirely both sides of the screen, any holes you leave will allow ink to get through. The edges are particularly prone to this, and will cause stray lines of ink to show up on your print. Do the best you can, and remember that on the top of the screen (the printing side, or flat side), you can extend the emulsion over the edge of the screen onto the frame, if that helps seal those edges.
Drying the Screen 
Make sure you dry the screen in a dark location, exposing it to light now will cause over exposure and ruin your stencil. If you have a dark closet or drawer where you can lay it flat and it won't be disturbed, that's fine. Other alternatives are to fashion an opaque box big enough to fit the frame in. If you're handy with wood, you can make one yourself, or you can just get yourself some sort of rubber storage container big enough for the frame. Most such containers are fairly transparent, however, so you can get yourself some nice thick black paint and coat the thing in a few layers of that to black out the light. Coating the inside is the cleanest, but depending on the kind of paint, it might be prone to scratching from the frame. Basically anything that will allow it to lay flat without the screen coming in contact with anything, and will prevent light from getting to it will do. You should also make sure it won't be exposed to too much dust, dirt, etc. It'll probably be in there for a day or two, so keep that in mind, especially in closets and drawers.
Your screen will take varying amounts of time to dry, depending on the emulsion, the thickness of the coat you put on, and the air conditions. In general, you're probably looking at about one or two days. I can't imagine it being more than that. A small fan blowing gently across it can help speed things up if you can arrange that.
As long as you're careful not to let too much light in, you can check on it once in a while by peeking into the closet/drawer/box. If it looks dry, you can check it by very gently touching the screen. Do this on the edge where it's less likely to cause problems if it's not dry. If it's not dry, it'll be at least a little tacky. Once it's dry, it shouldn't feel sticky at all. Also keep in mind that the top of the screen will generally dry faster than the bottom; be sure to check both sides before removing.
Exposing the Screen 
After we allow the emulsion to dry, we apply what's called a positive to the screen. That's positive, as opposed to a negative, like a camera negative. In a negative, all the colors are inverted, white is black, and black is white. For a positive, the colors are not inverted. For our purposes, it's a positive of the design we want to create the stencil for. In other words, we're creating a stencil which we will use to transfer a design onto our fabric (or whatever you're printing), so the positive should be dark anywhere you want the ink to be printed. The idea of the positive is to block light from getting to certain parts of the photo-sensitized screen. You can do this in anyway that gets the job done; if you want to print a circle, you can stick an opaque bowl upside down on the screen. More often, though, the positive is created on clear plastic, like acetate or a transparency, with a good opaque ink. We'll go into more details in "Positives".
With the positive in place, a large piece of glass that completely covers the pattern should be placed on top, to prevent the pattern from moving. Ensure there is no dust or dirt on the glass that may show up later as specks. Underneath the screen should be a black-non reflective material, or a piece of Styrofoam that will prevent light from bouncing up onto the underside of your screen, exposing the areas you don't want exposed.
Expose the photo-sensitized screen to light. Different photo emulsions require different exposures, but most can be done under a typical 60 watt incandescent light bulb in a few hours, with a makeshift reflector made out of a 10 or 12" pie pan. Stronger lights will require less time to develop, perhaps as little as 15 minutes depending on conditions.
Using unfiltered UV black lights, it only takes 5 minutes.
This is the recommended exposure chart for Speedball brand photo emulsion. Both of these charts apply only if you cut a hole in the center of a pie pan and place it on the socket before the recommended bulb, forming a reflector to reflect the light down on the screen.
With a 150W Bulb, Clear Incandescent:
Screen Size-------------Bulb Height---------------Exposure Time
8"x10"------------------12 inches-----------------45 minutes
10"x14"-----------------12 inches-----------------45 minutes
12"x18"-----------------15 inches-----------------1 hr. 14 minutes
16"x20"-----------------17 inches-----------------1 hr. 32 minutes
18"x20"-----------------17 inches-----------------1 hr. 32 minutes
With a BBA No. 1 Photoflood (250 watts):
Screen Size-------------Bulb Height---------------Exposure Time
8"x10"------------------12 inches-----------------10 minutes
10"x14"-----------------12 inches-----------------10 minutes
12"x18"-----------------15 inches-----------------16 minutes
16"x20"-----------------17 inches-----------------20 minutes
18"x20"-----------------17 inches-----------------20 minutes
Developing the Pattern 
Once the screen is properly exposed, you can remove it from the immediate light source and remove the positive. As soon as the positive is removed, you should start washing it out (under normal indoor lighting conditions, most emulsions won't develop in a matter of minutes while you're getting to the sink, etc., but don't dawdle too much, and certainly don't remove the positive, and plan to come back to wash out the screen at another time). This is done under regular tap water, nothing special required. If the screen was exposed properly, the unexposed portions should begin to become visible almost immediately under a strong flow of water, and a little vigorous scrubbing with your finger tips will have it fully removed in 10 to 15 minutes, or so. Once the stencil is totally washed out and allowed to dry, you've got yourself a stencil, perfect for screen printing.
A good positive can be made with dark ink on a sheet of transparent plastic. The two basic methods for this are by hand, and by machine.
By Hand 
The Ink 
Doing it by hand is probably the most accessible, but it can also be a real head ache. One of the first issues you'll face is getting a good ink. It is possible to produce a viable positive with permanent marker, but it will probably require double siding (see below) and it's more difficult to work with than some other alternatives, such as India ink. India ink is a rich dark ink, and is relatively quick drying. You can get India ink pens at most art stores in a variety of sizes, for probably around a dollar or two (US$) per pen. This is my preferred method for hand producing positives, but it's still a lot of work. I recommend you get yourself a bold pen, as well as a super fine. Different brands may mark this differently, but bold is generally about as thick as a standard Sharpie marker, and super fine is a about the size of mechanical pencil lead. If you're stocking up, you'll probably want more bold and super fine; if you're doing that much detailed work that you're going to go through the super fines fast, then you might want to think about skipping down to the next section. If it's available in multiple colors, black should be your first choice, since it will block the most light. You might also want to get one of each size in another dark color, such as dark blue or brown. This is useful for double siding, as I'll explain more below.
The Transparency 
Once you've got the ink, you need something to put it on. I recommend acetate, which you can probably buy at an art store in a little pad, like a sketch pad except with acetate instead of paper. If it's not acetate, it shouldn't matter, the only real requirement is that it be transparent, and that you can write on it with your ink. Some materials may accept the ink better than others. In my own experience, I've found that India ink on acetate never fully dries, and is therefore prone to smudging and rub offs. On the other hand, this can be a good thing as it's relatively easy to erase with a the tip of your finger, maybe slightly moistened. You'll probably run into similar problems with pretty much any combination of drawing ink and plastic, but some may be better than others, just experiment as you prefer, or ask someone more knowledgeable about plastics and inks.
The Pattern 
Ok, ink, transparency, check. But you still need to get the ink on the transparency, preferably in the shape of your intended pattern. The artistic type might choose to draw free hand on the acetate, but more likely you'll be tracing. Even if you want to draw the image yourself, it's probably better to do it with pencil on paper, first, then trace it onto acetate. For the non-artistic type, the image can come from anywhere you can think of. A simple graphics program (or a complex graphics program, if you prefer) can be used to generate some great mono color images that you can then print onto plain paper. Just remember, however many colors you print, each stencil is monochromatic, so your prints will be too. If you want to try multi-color printing, read this section, but I'd recommend you start with simple one-color prints first.
Also remember that you're going to be able to see right through the acetate onto the original image once you're tracing. This can make it difficult to see where you have and haven't already traced, especially if your image is the same color as your tracing ink (which is common if you're using black ink and printing images from a computer). To help this, consider making your images a different, probably lighter, color than your tracing ink. This will make it easier to tell the difference between your traced image, and the original image.
The only thing I have to say about getting the image is to keep copyrights in mind. If you're going to be selling your prints, don't use any copyrighted or trademarked material without permission of the owner. Even if you're not selling, you may want to check local laws about copyright infringement before using any such materials.
Once you've got the image, tracing is simple (in theory) since the sheet is transparent, just tape the image you want to trace to the back of the transparency, and have at it. As I said, this can be a real headache. You don't have a very precise eraser while doing this, and making precise marks with a bold tipped wet-ink pen can be a challenge. Fine tipped pens may help you be more precise, but they also tend to not apply quite as well, the tip is usually more firm than with a bolder pen, and tends to rub off ink you've already put down (which, incidentally, is the same problem you might face with a permanent marker). But a fine tip is good for doing outlines and fine details, and use the bolder pen to fill in larger areas. If you really want to go all out, get yourself some in between sizes, as well to help fill in thick lines, or thin spaces. Just remember, however quick drying the pen claims to be, it probably isn't meant for plastic, and probably won't dry all that quick, if ever, so be careful about smudging. You'll typically want to work left to right for right handers (and right to left for left handers), and then top to bottom, so you don't accidentally smudge your earlier work with your hand, wrist, arm, or what have you.
Double Siding 
When you've finished tracing or otherwise producing your image on the transparency, remove the original image from the back and hold the transparency up to the light. You'll probably notice that quite a bit of light still comes through the ink, and you may even notice places where you didn't trace at all, or where the ink got completely rubbed off. All of these little imperfections can cause trouble when you're exposing your screen, because if too much light gets through, the emulsion will set and you won't be able to wash it off, which means your stencil will have little marks where the ink doesn't go through when it's time to print. You can fix these up when you spot them, but it can be difficult to find them all, and there's always a chance that it'll get rubbed off again before you print. I've found that the best way to deal with this is a process I call double siding, which simply means you retrace the image on the reverse side of the transparency. The idea is that if there's a small imperfection in the image on one side, there's a slim chance that there will be one in the same spot on the other side, you at least you've got a good fighting change. Furthermore, double siding basically doubles the opacity of your image, blocking out more light. This is very useful if you've found yourself prone to over exposure.
Of course, double siding does produce it's own little set of hassles. For one thing, all that work you did tracing the image, you've got to do it all over again. Second, you've get ink all over what is now going to become the back side of the sheet. You've got to be very careful not to smudge it too much, or rub it clean off. Remember, though, the image is already there, you don't have to trace from the original again, so a good defense against smudging is to tape a sheet of white paper over your first trace, so that as you're tracing the back side, the wet ink will be somewhat protected. It's not perfect, though, you can still smudge even like that. Fortunately, you're double siding so smudging isn't as big of a concern. Lastly, remember how I said it can be hard to distinguish the original from your trace if the colors are too similar? The same goes for double siding. If you're using the same ink on the back as you did on the front, it can be even more difficult to tell what you've already re-traced. holding the image up to the light can be helpful for seeing this, but what a hassle. An alternative, as I mentioned briefly above, is to use two different colored inks for the front trace and the back trace, this will make it easier to tell the difference but remember, the whole point of double siding is to block out more light, so you have to make sure both inks are relatively opaque.
Saving Positives 
You may likely want to save your positives once you're done with them. Especially if you're planning to reclaim the screen, you might want to recreate the stencil later, and you certainly wouldn't want to have to go through all that again. Unfortunately, the ink on plastic problem can get in the way of this, so just be real careful about storing them, don't put them somewhere that they're likely to be shuffled around a lot, or have things set on top of them. There are probably ways of treating them to make them more permanent, or even to coat them with something. Go to your local art store or hobby store and see if they have any suggestions. Just remember: the whole point is to block light from the inked spots, and let light through the rest, so don't do anything that's going to conflict with that, for instance some kind of varnish that clouds up when it dries and blocks a lot of light from getting through.
By Machine 
This isn't as intimidating as it may sound. Machine in this case just means a computer and printer, or a photocopier. The idea is to simply use one of these devices to print your pattern directly onto a transparency.
Office supply stores usually sell transparencies designed to work with various printers. You want to use the correct transparencies/acetate with laser printers, if you use a regular acetate, it will melt in the printer. This is a low quality method to create film positives.
Good quality film positives can be obtained by using ink-jet printers. If you combine a dye ink Epson printer such as the Epson Stylus 2400 with the correct films such as AccuBlack or AccuArt the results are fine for commercial screen printing. For good results you are looking for high opacity and a high contrast between the film and the ink laid down.
Alternatively, many copy shops should be able to copy a pattern onto a transparency. This may be quick but the opacity is low. Between laser printers or paying a copy house to do it, I don't know which is more expensive in the long run.
Which ever you choose to use, make sure the image is going to block out enough light. Typically, this kind of printing or copying is intended for over head projectors which don't actually require very opaque images. You can try printing a sample to see how opaque it is. If you've done a lot of photo emulsion stenciling, you can probably get a good idea just by looking at it against the light of whether or not it's dark enough. If you're not sure, you can experiment with it, and just be prepared to waste the stencil if it's not dark enough, or you can maybe waste a little money and play it safe. There's a few things you can do to make it more opaque. First, if you're at a copy shop, ask the clerk for suggestions, they may have special toners that are more opaque, and they can probably increase the contrast to make a darker image. If you're printing at home, you can play with your printer settings to try making the image darker. An alternative for either copier or printer is to do double copies of all your positives, and just make sure they're well lined up before using them on your screen. You can use clear acetate desk tape to fold them together since it won't block any extra light. You just have to be real sure you get them lined up right or you're image will end up kind of funny.