Exercise One: Sit or lie comfortably with your eyes shut. Relax your body. Breathe deeply and still your mind. Pictures will continue to pop into your head. Choose one of these and stick with it. Let no images intrude other than the one you've chosen.
Keep all thoughts revolving around the image. Retain this picture for as long as you can, then let it go and end the exercise.
When you can retain one picture for more than a few minutes, move on to the next step.
Exercise Two: Decide upon an image to hold and retain it within your mind. You might wish to have it physically present and study it first, memorizing each detail- the way shadows play on it, its textures, colors, perhaps even a scent. You might choose a small three-dimensional shape, such as a pyramid, or something more complex such as an image of Aphrodite rising from the sea or a ripe apple. After studying it thoroughly, close your eyes and see the object before them- just as if your eyes were open. Don't look at the object again with your physical eyes but with your magical imagination- with your powers of visualization. When you can hold this image perfectly for 5 minutes, move one.
Exercise Three: This is more difficult, and is truly magical in nature. Visualize something, anything, but preferably something you've never seen. For instance, let's use a vegetable from Jupiter. It's purple, square, a foot across, covered with quarter-inch green hairs and half-inch yellow dots. This is just an example of course. Now close your eyes and see- really see this vegetable in your mind. It's never existed. You're creating it with your visualization, your magical imagination. Make the vegetable real. Turn it over in your mind so that you can see it from all angles. Then let it vanish. When you can hold any such self-created image for about five minutes or so, continue on to the next exercise.
Exercise Four: This is the most difficult. Hold a self-created image (such as a vegetable) in your mind with your eyes open. Work at keeping it visible, real, a palpable thing. Stare at a wall, look at the sky, or gaze at a busy street, but see that vegetable there. Make it so real you can touch it. Try having it resting on a table or sitting on the grass beneath a tree. If we're to use visualization to create changes in this world, not in the shadowy realm that exists behind our eyelids, we must practice such techniques with our eyes open. The true test of visualization lies in our ability to make the visualized object (or structure) real and a part of our world. When you've perfected this exercise, you're well on the way.
~ from Wicca: A Guide For The Solitary Practitioner
by Scott Cunningham