Flat on your back, with your neck supported.
(Some posts, I like to give the simple answer for those without the patience to read a couple of hundred words.)
Notice that I said ‘neck’, not ‘head’, supported.
You don’t necessarily want to focus on raising your head, which will just curve your neck into an unnatural position.
One regular pillow properly placed (not seven) will do the trick. Yes, there are specialty neck pillows out there, which are designed to fit directly into the neck area. I haven’t tried those yet.
You don’t want to be on your sides or your stomach. Here’s why:
When you’re on your stomach, you restrict full inhalation & exhalation.
Deep breathing is a key component of your health, for many reasons.
When you go to sleep on your side, it’s almost inevitable that you’re going to impinge something (like your shoulder, for example) by putting too much pressure on it for too long.
Now, the key is to begin your sleep on your back.
Yes, you will move throughout the night a bit. Yes, at times, you may end up on your side or stomach.
But by practicing falling asleep on your back, in my experience, you will begin to get the deepest possible sleep, with the least amount of malignment.
Keeping your body aligned and well rested makes the morning far more productive and far less painful.