Meditation. Sit or lay in a comfortable position. You basically wanna get into a deeply relaxed state, to the point where you barely have any thoughts, some say no thoughts but personally I believe you always have thoughts. Keep going like this until you feel slight "vibrations" in your body. It might not specifically be vibrations though, iv heard it described as many things.
Before you get this far when you first start out, you might fall asleep. I did. I still do sometimes.
Then try to control the vibrations. That is basically the key to astral projection/OBEs. Its a little weird to explain, but once you get that far you can leave your body. It is advised though, that you start out small. Move your arm first, then the next day your leg. Not your physical leg your astral leg. Then when your ready, you can roll out of your body or float out.
Its important to remember not to freak out or get excited. That pulls you right back in.
It is hard to do. But with dedication, practice, and patience you will be on your way.