Helping others to understand things, and enjoying doing so, is an excellent way to know that you'd probably be interested in teaching --- anything, much less a CHL course. When will you know that you're ready to do that? It depends on many things... your personality... your ability to transfer knowledge by demonstration or explanation... your ability to help someone that seems to "not get it" even after numerous attempts.
I've been teaching in the public schools for 14 years, now. My wife had been suggesting I move into education as a career for a long time before I finally made the jump. In my previous career as a paramedic, I also held an EMS Instructor's license, and helped teach EMT and paramedic continuing education courses, plus mentored students who were "riding out" on an ambulance for practical experience. But, even before that, I've been involved in teaching amateur radio license courses for about 30 years, and also did some photography instruction in classes on-base while I was in the Air Force during my 20's. So, as you can see, I've been in and around education pretty much all my life, so getting a CHL instructor's certification was a natural extension of my interests.
As far as having to become an expert in firearms, you don't have to be that to be a good and effective CHL instructor. You do need to have a good foundation on the basics and practice of firearms useage - after all, how can you instruct someone on how to fire a gun effectively if you can't do it, yourself?
(To those that say that a CHL instructor is NOT supposed to be teaching a student how to fire a gun - you're right, if we're talking about the Texas CHL course. But, before you start teaching "the course", I would assume that most instructors inquire about a student's abilities with a handgun, and probably offer some additional tutoring in shooting if it's needed - certainly that's what I have seen from the instructors whose classes I have observed, and do that with my own students).
The course that DPS uses to certify instructors will teach you about teaching a CHL class. It will explain the laws that your students need to know, and will teach you how to go about teaching the CHL class - what to do when, and how.
You will also need to have been certified by either the NRA, the law enforcement certifying agency in Texas, or some other nationally recognized shooting organization as an instructor. This supposedly makes sure that you can teach students how to shoot, to do so safely, and effectively. Most people I've heard from take the NRA Basic Pistol Instructor's course. And to do that, you also have to have the NRA Basic Instructor's course.
I'd also suggest that you need to enjoy shooting to become a good CHL instructor. You need to be a people person, who likes to see people succeed at tasks and who is willing to contribute to someone else's knowledge and abilities for little too no monetary rewards (I don't think you'll find too many CHL instructors that are making a fabulous living teaching just CHL classes).
If all of this sounds like I'm preaching to the choir as you're reading it, you might be ready to take the step of getting into the Instructor's ranks. Gotta do a bit of prep work before applying to DPS, though, so if you need any help finding an NRA Instructor's class, start by going to
http://nrainstructors.org" onclick="window.open(this.href);return false; and click on the "Become an NRA Coach, Instructor, Training Counselor or Range Safety Officer" link on the right side of the page, to start looking for a class near you.
Good luck!
Wayne