1. Relax your body completely. I can only do this lying down, but some people can do it sitting in a chair.
2. Relax your mind until the only thought that you have is this sense of that you "should be" somewhere else, like 2 feet in front of where you already are.
3. Decide that's not going to happen without giving up on it.
If you understood all that correctly and got the right balance, then in about 20 minutes, your astral body should naturally begin to wriggle out of your physical body. That can be the scary part, because it's noisy and you're moving without moving, but if you can continue to be relaxed during that transition, then you'll find yourself in an exact copy of the room that you are meditating in...except you don't have a body.
Now you can haunt the world like a ghost, without the harsh prerequisites! (Okay, this was a kind of harsh prerequisite, but it's not AS harsh as most ghosts, you know?)
There's another method that I consider closer to Carl Jung's "Active Imagination" exercises that might be more useful, though.