Its pretty straightforward to become an instructor. You take the BIT first (Basic Instructor Test). That course teaches how to teach. Then you take the discipline specific course/test. The NRA is a pretty big fan of lists, so take notes outline style. You don't really need to study, they give you what you need and most of it is basic information. The shooting part is pretty easy. If you can shoot groups inside the 8 ring, you'll be fine. If you can't, you shouldn't be teaching
The hardest part is sitting for 2 days. Good luck, we always need more people teaching safe firearm use.
NRA Certified Instructor Pistol