What is visualization? Put simply it's the act of holding an image or several images in your mind with clarity. That's all it is.
Doing it is just as simple.
Try this. Take a common object - pencil, pen, etc. - something that is interesting to your eyes and you feel comfortable with.
Hold it in your hands. Focus on it and nothing else. Rotate it slowly so you can see all sides of it.
Now close your eyes. Try to picture yourself rotating the object in your mind. Do your best to recall as many details as possible. Open your eyes if you're unsure about something in particular. Try to build the object in front of your eyes while they're closed.
It'll take time but not as long as you think. The important part is to do this every day in some small way. Like a muscle this ability will grow stronger the more you use it.
Once you've done one object - try for a second one. Two at once, then three, then four, etc. Try re-building the room you're in in your mind. Re-organize the furniture in your mind so the room has all the same parts but is completely different.
Don't be troubled if you can't visualize exactly right down to the scratches on the wooden coffee table. That will come with time and practice. Just keep working at it.
After you're comfortable with visualizing objects, rooms, whatnot, then you can move on to visualizing goals for yourself.
Start with a little bit of motion in your mental scenes. Put yourself into those scenes.
Again, little by little change the scene you're visualizing to what you want.
Visualize yourself learning something you always wanted to. Or getting a new job that pays more or has more perks. Visualize yourself on vacation - a good anti-stress method by the way.
Focusing on these goals using visualization will make them far more real to you and much easier to work towards.
Another exercise for increasing your visualization skills is through art. Drawing, painting, sculpting and other forms of art are all ways of creating links between your initial perceptions of things, and how well you can recreate those perceptions. For example: Try to sketch an object without looking at it. When you have gotten as far as you can, try to reconcile the drawing with the original image. How much of what you drew was merely what you expected to see instead of what you actually saw?
A good drawing exercise for training your perceptions is to try to copy another drawing upside-down. Take a dollar bill and turn it upside down. Then try to copy it (not tracing, just copying it line for line). This exercise helps you to become an accurate scribe of what you see, instead of merely what you think you see.
The goal is to develop your ability to record specific visual information. In many cases it is better to amalgamate information instead of remembering every detail. These exercises will help you develop the ability to choose how your brain will store information.