There are many different types of certifications for firearms instructor. For instance in CA, the BSIS certifies firearms instructors for security guards, DOJ (IIRC)for CCW.
I'll deal with NRA certification:
Since you have read the prerequisites on the NRA site, you need to find an NRA Training Counselor who is certified in the area you want to teach, e.g. Pistol. There are 6 different disciplines, including pistol, rifle, shotgun, RSO, reloading and Personal Protection.
You attend a course (the NRA pistol instructor course is 17 hours/2 days), demonstrate proficiency with the firearms and pass a couple of written tests then wait 4-6 weeks for your certification to arrive in the mail. At that point you are good to go.
If you have more questions, email or PM me. Instructing can be a lot of fun and very rewarding.
