It is actually an attention focusing intent driven technique. There are lots of methods, but fundamentally the idea is to allow your body to fall asleep without switching off the mind.

Robert A. Monroe is really the Grandfather of modern day Astral Projection. His earliest method was just to concentrate at a spot in his mind outside the body as his body was falling asleep.

Over the years this evolved into a technology aided system using Hemi-Sync (which to this day I don't use).

The idea is simple, focus on being consciously awake and allow the body to fall asleep naturally.

Interupting your sleep cycle ie... sleep 4-5 hours wake up for 30 minutes physically out of bed. Go back after bathroom etc and no more then 1/2 cup of fluid (non-cafinated).

Then try to focus on being awake on a spot and let the body fall asleep. Don't focus on the body and allow all the natural changes of sleep to occur.

Takes a bit of practice, but if you are sincere and have the right intent and attitude it will happen.

You are consciousness. Being conscious is natural. Even during physical sleep.