After learning the basics, what's most important?

It sounds like you both are getting the idea, and I hope you can see it's quite simple. It's more like "remembering" what to do than "trying" to do something new.

Once you have the basic idea, I believe your level of performance is a function of your focus, concentration, intention, and commitment to the target. As you go to shoot, and you're all set to start the UpForce, connect to it, align with the basket, pull the trigger, etc., note how "connected" you are to the basket before, during and after the shot. I feel if you are totally connected and trusting and focused, your shots will just drop like pennies from heaven, over and over, dead center, Swish. The Follow Through is a big part of this, as it helps guide the ball in the latter part of the shot. Note how strong your intention is in the Follow Through.