First you take the basic course of the discipline. (NRA Basic Pistol, BAsic Rifle.. Etc)

Then you contact a TC - A training counselor that is certified to train instructors. You will take the instructor course with him.

After finishing that course, You will get your credentials in about 4 weeks.

Honestly though, there are tons of decent shooters out there that think they can be instructors. The simple truth is that this is a teaching job, if you are not a good teacher, this isn't for you. Being a good shooter is secondary to the ability to effectively teach.