My wife envies my ability to decide to go to sleep and then just do it without further ado.
But I am here to tell you that this is not a natural ability; it is an acquired skill, and you can learn it too.
Here is my secret for falling asleep at will:
Get comfortable, that is, get in a position and situation where you don’t need to shift around to stay comfy. You won’t fall asleep if you have to turn over every few minutes. This might require some body awareness.
Turn out the lights and close your eyes. Eye-closing is not optional.
Step 3 might be the hard part. Meditation might help to learn this. I just started reading Eckhart Tolle’s “The Power Of Now” and so far he’s doing a good job of talking about this.