You might want to use the NRA course locator
https://www.nrainstructors.org/search.aspx
The basic pistol instructor course will take you a weekend (two, 8 hour days plus some time for lunch and breaks) and probably run you in the area of $300 to $350. Then you can see where you want go from there.
The NRA then offers a number of other pistol instructor courses including Personal Protection Inside the Home, Personal Protection Outside the Home and Concealed Carry.
Which, if any of those, would qualify to teach in your state varies by state.
And, as a poster alluded to earlier, the NRA offers two range safety officer course: RSO and Chief RSO. The latter allows you to teach their RSO course and, in theory at least, supervise RSOs.
Some good info in the posts here. Be warned, there's some up-front investment: your time, course costs, travel, teaching aids, liability insurance (check out Lockton Risk), etc.
The entire process can be a bit of a long row to hoe -- unless you enjoy the classes and the camaraderie, in which case it can be quite fun and a chance to meet some good people.
And, as others have noted, being able to do is not the same as being able to teach. The latter can be studied, though, especially by working with and taking instruction from masterful instructors and training counselors (the NRA's instructors of instructors). Enthusiasm, tolerance for your fellow human beings and an open mind are all big pluses.
Running your own business ... a whole 'nother skill set.
I have several of the NRA instructor certs: basic pistol, rifle and shotgun, PPITH and PPOTH, RSO and CRSO, Refuse to be a Victim, and Level I Shotgun Coach.
That said, I've never run my own biz nor wanted to. Been an extra hand to instructors who do run their own businesses and been a volunteer for various shoots/matches.
I have enjoyed it, but I'm kind of retired from that at present. Were I to return to it, I would re-up my liability insurance.
Good luck.