When I have a intramural game or I want to get something out of shooting around, I have a small routine.
I start by shooting little bank shots right on key like 4 feet away from both sides, alternating sides. I SLOWLY work my way out, but still banking it. This warms me up a little, gives me a feel for the ball, and gives me a feel for how the glass reacts to being shot at.
Then I'll stand right beneath the hoop and shoot it straight up with just my shooting hand. Then I move my feet back and do it again. I keep shooting until I'm shooting a few feet past the three point line at the top of the key. I don't move on until I make the previous shot.
I don't start shooting threes until I feel good. If I never feel good, I never shoot threes. If I need to get the shot back I'll do step 2 again or shoot free throws.