When you sleep, your soul leaves your body. It literally leaves our three dimensional physical reality. This is caused by your pineal gland (the most important part of your brain) releasing Dimethyltryptamine; also known as DMT. DMT is what propels your soul out of body when you sleep, and when you're about to die or have a near death experience. The thing is, you do not have control over what your soul does when it leaves your body while you sleep,your subconscious controls it.
The difference between normal sleep and astral projection is, when you astral project, you are able to consciously control your soul, and where it goes. A key ingredient to this projection technique is an invisible, imaginary rope hanging from your ceiling. This rope will be used to exert dynamic pressure at a single point on your astral body to force its separation from the physical. Reach out with your imaginary hands and pull yourself, hand over hand, up the strong, invisible, imaginary rope hanging above you. You will feel a slight dizzy sensation inside you as you do this. This feeling of vertigo will intensify the more you pull on the rope. Keep climbing, hand over hand, ever upwards, and you will feel the vibrations start. Your whole body will seem to be vibrating and you will feel paralyzed. Concentrate single minded, on climbing your rope. Don't stop.
Next you will feel yourself coming free of your body. You will exit your body in the direction of your imaginary rope and will be hovering above your body. You're free at last!