Asklemmy

There's a couple different aspects, and a variety of techniques. First part is to create a habit of something. Previous poster seems to be using a coin flip. For me, I like to make a habit of pinching my nose closed and trying to inhale through it. Obviously, this doesn't work when you are awake, but will work if you are dreaming. It may take weeks of developing this habit (do it every time you think of it), before the habit makes it into your dream.

Realizing that you are dreaming may wake you up the first few times, but eventually, you will be able to stay asleep while aware you are dreaming: aka lucid dreaming.

The other part, aside from developing that habit, is prepping yourself before sleep. Again, you'll find different techniques, but here's the one I favor. Lay on your back in bed, eyes closed. Focus on your legs, for 10 breaths, then your arms for 10 breaths, then your sense of hearing for 10 breaths, then sight (eyes still closed) for 10 breaths, sense of touch for 10 breaths, etc. Keep cycling through until you fall asleep. This primes you to be "located" in your body once you fall asleep. If you're lucky, you'll perform your "habit", realize you're asleep, and have full control of your body! Good luck :)