This is done just like it sounds. To get started lay down on your back wherever you want. I prefer a bed, but it depends on the height of the ceiling. Sometimes it is easier to lie down on the floor.
It is o.k. to get a pillow and place it under your head. The point to the drill is to promote good shooting form without having to worry about making or missing a shot.
While laying on your back place the ball in your shooting hand and get the ball in position to be shot into the air.
Make sure that you have your elbow in and that your off-hand is only being used as a guide hand. Keep the ball from getting too deeply into the palm of your hand.
When the ball gets into your palm players tend to flip the ball instead of truly releasing it the way it will naturally feel. Shoot the ball into the air. Pay special attention to the rotation on the ball.
If the ball is knuckling (no spin) or spinning sideways then you have an issue with your form and it needs to be corrected. If the ball is coming off sideways it is typically a problem in one of two areas. It could be the guide hand is too much on the ball or the ball is coming off the incorrect fingers of your shooting hand.
The ball should come off your middle and pointer finger with each shot. When it slides to the outside of your hand it causes a side spin. The ball knuckles typically when the thumb of the guide hand gets involved. Make sure that you aren't using your guide hand in any way to propel the ball in the air. You will notice that you will get tired doing this drill and that your forearms will burn a little. That's good. But the key to the drill is repeating correct form over and over again. You can judge that by the rotation on the ball and where the dirt shows up on your fingers.
Larry Bird Warmups
Larry Bird used to go out hours before a game and start his ritual. Now a lot of us don't have that much time, but we should try and emulate Bird in anything we do. Larry Bird was an amazing shooter who mastered muscle memory with his shot. It wasn't the prettiest shot but it had two things going for it over all others.
One, Larry Bird was a two eyed shooter. He could see the target with both eyes as he released the ball. Second, Larry Bird had the ball go off his hands perfectly. I had the pleasure to be up close and watch him go through his routine and that ball comes off those two fingers perfectly. It was like he was throwing darts instead of shooting.
Larry would start his shooting practice 5 feet away and work his way back. He spent the most time at the foul line just working on form and fundamentals. He wasn't out there dribbling behind his back or laughing at all. He was very serious and focused. Larry didn't just settle for making shots, he wanted to swish everything. If it didn't feel right at the distance he was at he wouldn't move at all.
As I watched him he didn't get to the 15 foot range for 45 minutes. He just shot the ball over and over focusing on his form. If it was off at all he would stay in the same spot until he felt better about it. The reason I included this as a drill was because none of us are better than Larry Bird and he did things the right way. He mastered the basics yet still felt the need to practice correctly. It is a great way to get comfortable with your shot and I highly encourage both these drills for anyone truly looking at improving their form on their shot.