First, you need to define what you mean.
At the most basic level, you need to know more about the subject than the person(s) you teach.
My dad was a great instructor, though I had no formal "classes" with him.
I was an instructor to my kids, from the point that I could make myself understood to them.
Now, if you mean "NRA Certified [insert discipline here] Instructor, you will need:
1) find a Training Counselor in the discipline
2)Successfully complete Basic Instructor Training
3)Successfully complete the training for the discipline (Basic Pistol/Home Firearm Safety is about 26 hours)
Part of the above includes displaying proficiency with the guns involved.
There is no "Shoot a 90 point target" cutoff for these, as that's not what you're teaching.
If you're interested, there are many TCs here (ScouterRick, Jon Green at GOAL, for instance) that can set you up.
More important than lots of knowledge, or experience, is attitude. My son wanted his BP/HFS Instructor Certs. for his 13th birthday; he just added Shotgun. Not to brag about my kid, but he's a good instructor, and he's still just 14. You have to want to do it.