What is Astral Projection? Well, it is the "sending out" of your spirit body from your physical body. This is only one method, though there are many, many more out there.
To begin, you will need a place that is quiet, with a very small amount of light (but not total darkness). You must not be interrupted, either. The place should be comfortable, but not so much that you fall asleep (you can do this while lying on the floor, an hour or so before you normally go to bed).
You should have no restricting clothing on, including watches, jewelry, etc. If you are wearing something tight, it will be extremely difficult if not impossible for you to achieve separation. Also, during the three hours before you begin this, you should not eat anything, nor should you have drunk anything with Caffeine or Alcohol in it. If you have, your system will be either sped up, slowed down, or involved in digestion, which will make it very hard to achieve separation.
And now that you have your place and are physically ready, you should clear your mind. As you progress through the following steps, you will become susceptible to random thoughts, you must maintain control of your thoughts or you will have to start over. Take a deep breath. Now, either aloud or to yourself, say, "I am going to go to my Alpha State".
Now, begin to visualize, in order, the colors Red, Orange, Yellow, Green, Blue, Violet. To do this, visualize an object that is that color or just the color itself (a stop light, or tee-shirt, or some other thing you are familiar with will work well) When you are satisfied that you have actually seen the color in your mind, even for but a brief second, go on to the next color. When you have gone through all of them, say to yourself, "I am now at my Alpha State".
You should now be very relaxed, you are basically in the state your body goes into while you sleep, although you are remaining conscious now. Now, concentrate on your big toe (either foot). It should begin to tingle. If it does not immediately tingle, keep concentrating, it will.
When it begins to tingle, concentrate on the other foot's big toe. And when it tingles, move to the soles of your feet, then the top. Do not worry about the areas that you have already gotten tingling, they will continue to tingle after you move on. Now, move the tingling up each of your legs, up to your waist. Then, starting with your fingers, do the same thing to your arms.
When both your arms and legs are tingling, bring the sensation up from your waist through your back and up to your shoulders. Then bring it from the top of your arms across your shoulders, then down your chest. Keep concentrating until your entire chest tingles. Now every part of your body except your neck and head should be tingling. If there is a spot that is not, go back to it for a little while.
When you have everything but the head/neck tingling, slowly take the feeling up through your neck. Now comes the hardest part, relax all the muscles of your head, even your lips should be relaxed. Bring the sensation over the back of your head, until your scalp is tingling. It may take a while, but when that is done, send the tingling sensation over the rest of your head.
Your entire body should be tingling, as if you are plugged into an electrical outlet. If it is not, continue to concentrate until it is. It may take a while at first, but gradually it becomes easier and easier.
Now, to separate, think of a place you would like to go, the closer it is, the easier it will be. Visualize what the place looks like if you are familiar with it. Think of going there, being there. By mere strength of will, you should be able to separate from your body. Continue willing to go until you do indeed separate. Some people hear a metallic 'Click' when separation is achieved.
Move about, simply wish to move. Not only that, but you do not have to walk, you do not have to go around objects. You are no longer limited by your physical body. You do not have to walk, you can, actually, fly! Whenever you want to return to your body, simply wish to be there. Going back is the easiest trip!
Now, when you return to the body, slowly open your eyes. Do not move about suddenly. You will probably feel a little disoriented, and like you just woke from a deep sleep. Slowly move your arms around, and bring your legs up (bend the knees).
After you have moved around a bit, stand up and walk around a bit. Now you are back in your physical body, so go about what you would normally do! Congratulate yourself on a successful trip! After you have actually separated, it will become increasingly easier and easier for you to do it.