You can find detailed instructions in dozens of books on magic -- one I recommend is Apprenticed to Magic by W.E. Butler. The very short form -- and you'll want to read at least one book on the subject before trying it -- is that you build up an image of your body in your imagination, as though it's standing a short distance in front of you, and you then transfer your consciousness to that image by an act of will. It takes practice, and exercises in concentration and visualization are good preparation.

The only real dangers you face in doing astral projection are getting too obsessive about it, on the one hand -- it takes a lot of energy to do, and you can exhaust yourself by doing it too much -- and mistaking astral perceptions for absolute truths on the other. I'll be covering that in this week's post on the blog.