Guitar/Stringing the Guitar
Aside from the physical shape of the guitar body, strings are the most important thing for determining the sound of a guitar. New strings sound bright and full, while old strings tend to sound dull and dead. Many guitarists believe that strings should be changed regularly, not just when they break. This is because sweat and dirt corrode the strings, and over time this degrades their sound quality. Other guitarists believe that new strings sound much worse than old ones, feeling that a string's tonal quality only improves over time. Individual string quality may vary drastically from string to string.
When one breaks a string, all of the strings should be changed at once. This is especially true if the newer string is of a different brand or gauge. The string's manufacturing process, thickness and age all affect its tone, and one new string being played with a bunch of old strings can make your guitar sound strange. Players should be advised that guitars are usually set up for a particular gauge of string. The guitar will still function fine with a different gauge of strings, however for optimal sound, the guitar may need to be adjusted. See the chapter on adjusting the guitar for more details.
Because there are several different types of guitar, and each type is designed differently, each type has its own method of stringing. The type of strings you use mostly depends on what style of music you play and how long you've been playing. Thinner strings are generally preferred by beginners, but many experienced players prefer the feel of thin strings over thicker strings. Please see the guitar accessories section for details on different types of strings.
The first thing you always need to do when stringing a guitar is to take off the old strings. You should never just cut the strings of a tuned guitar in half, because the sudden release of tension on the neck can damage the guitar. Instead, always turn the tuning pegs to decrease tension, until the string is so loose that it doesn't produce a note when struck, then cut or unwind them. In most cases, the string is bent at the end where it was inserted, to insure that it would stay during tuning. Unbend the string, then pull it out of the peg hole. If the peg end of the string is too bent or curled from the winding, cut the string on a straight part of the string. This will make it easier to remove the string from the hole at the other end and reduce the risk of scratching the body or the bridge while trying to get it out. Slide the string out of the bridge at the bottom end of the guitar. Some people string one at a time to make sure the neck sustains tension, or they just take all of the strings off at the same time.
Stringing Acoustics 
Standard guitars typically have a ball-head peg at the bridge section. This peg has a hollow shaft, with a groove that allows the string to come out from the peg.
Typically, the process is as follows:
Unwinding the string
- Pick one of the strings, usually either the first or sixth string, and begin loosening it. If you have a string winder, put the rectangular box over the tuning peg and unwind the string.
- Once the string is loose enough, pull the peg out of the bridge. If you have a string winder, it will have a notch that can fit underneath the head to pull it out.
Attaching the string
- The guitar string should have a ball (or cylinder) end. Put that into the bridge hole.
- Push the ball-head peg back in. as you do, pull on the string so that the peg can hold the string tightly on the bridge end.
- Find the hole on the winder, and place the string through it, leaving about 5cm out on the other side. For the thicker strings, it is recommended to bend it a bit.
- Use one hand to hold the string so that the section between this hand and the peg is tight. Wind one wound.
- Check the tightness again, and try to divert the string so that it wind UNDERNEATH the winder hole. Then wind it until you have the string encircle the machine head two to three times.
- Tune from here.
Twelve String Acoustic 
It has the same principle to the sixth string. But every two strings were tuned with the same sound, one octave apart. *(RDT)
Classical Guitar 
To unstring a classical guitar one method is:
- Loosen the string by turning the tuning peg
- Then at the bridge push the string back into the hole a little, this will loosen the "knot" enough to unknot and pull the string out of the hole.
- Then feed the string around the peg loop by loop until the last hoop which is inserted through the hole in the peg is available, push the string out of the loop, then pull the loop out of the hole.
To string a classical guitar one method is reverse of the unstringing
- Bend about an inch of string at one end to form an open loop, push that through the peg hole, wrap the other end of the string around the peg and through the loop, then pass it down the guitar body to the bridge and into the hole there.
- Loop back to the neck (about two or three inches) and twist back around the string, then you can put two or three twists in which should end up on top of the bridge, pull the string from the middle of the guitar to draw the twists taut.
- Then wind the peg to tighten the string. You should take it easy when tuning up for the first time to give the string time to "settle in", you may also find that the string may go out of tune easily for a day or two as it beds in.
Stringing Electrics 
For the 6th string (the low E), take the string out of the package and insert the end through the bridge of the guitar. Pull it all the way through until the ball at the end of the string stops it from being pulled further. This is optional: Make a kink in the string to insure that it will not slip away from the turning of the peg, (usually about one or two inches from the peg). Wind the string around halfway and insert the end through the hole. Pull the string to add tension, so the string will stay around the peg during tuning. Turn the tuning peg to increase tension until the string is around the desired pitch, to make certain it will stay on properly. Check that the string is in the notch in the nut and the bridge, if it is not, decrease tension on the string until you can move it into the notch, tune it back up. Do this for the rest of your strings and you are done!
String the low E and other strings as mentioned. Align the tuning peg's hole with the direction of the string and slip it through the peg in the direction of the headstock. Facing the guitar with the headstock to your right, pull the string taut with your left hand.
With your opposite thumb and forefinger, twist the string in an "s" at the twelfth fret so that it touches both sides of the twelfth fret. You will have to let some of the string out to do this. This method tells you the optimum length of the string to wind around the tuning peg.
Hold the string with your right hand below the tuning peg so that the pointy end is sticking out the other side. Slowly tighten the peg so that the string is winding on the INSIDE of the headstock -- inside right for E A D, and inside left for G B E. Allow the string to wind once underneath itself, and then wrap it over top of itself the rest of the way. Make sure you hold tight as you go so that there is little slippage later.
If possible, hold the string with your right thumb and middle finger while regulating the pressure on the string with your right index finger.
- Note that taking off all strings at once is not recommended if you have:
- a floating tremolo system (e.g. Floyd Rose II), which can be difficult to get the tremolo angle back to the right level when restrung;
- a bridge which is not fixed (one that will just fall off when the strings are removed)
- Try not to bend the string in the same place excessively otherwise the string will break at the bend
Twelve String Electric 
Online Resources 
- Cyberfret's tutorial - Perhaps the most common way to string the guitar. Covers acoustic and electric.
- Alan Horvath's tutorial - A different way to string the guitar. Covers acoustic, electric, and classical.
- Uncle Tim's How To String A Guitar - How to string a guitar so it never falls out of tune.
- Shredaholic's Tips For Stringing Guitars Other useful tips and observations on stringing electric guitars.
-  Detailed guide to stringing an electric bass guitar