Once you get the physical aspects of the position down, for a QB the game is probably 90% between the ears in my opinion. You have to know everyone’s assignment, the protection, and most importantly be able to read the defense.
Now you have a pre snap generalization of what the defense is going to do but all of that can change during the play. The first place your eyes should go is the safeties, then the corners, then ID the men in the box/defensive front. That should give you a basic idea of the coverage and the pressure coming but doesn’t necessarily mean that’s what they will stay in. They could rotate the safeties and bring more pressure, blitz a corner and etc. So a lot of this you are going to have to pickup in your drop and while you are making your reads. The only way to get good at this is reps. The week before a game I would always go through every single play and go through all of the different scenarios the team we were playing could run against us. Every night. The reps you need, can’t just be done in practice. You obviously do the reps in practice but you need to be studying film like a mad man and going through your reads every night. The film study has a lot to do with their tendencies. When do they like to use certain coverages? Do they have any tells of when they are bringing pressure? Who is their best player? A lot of things can be answered through film. If you don’t watch film you won’t be prepared and able to recognize anything the defense is doing. You need to prepare for anything they can throw at you. Once you get into the season you probably won’t be developing the physical side of your game too too much, so the mental side is number 1.