Not Cathe here, but I can give you a couple of pointers.
For those that are already GX instructors can you teach a 32 count class? One of the good and bad things about teaching step is it requires you to be precise about when you cue. When you are teaching there are a million things going on and it can get overwhelming to track them all. As I got into instructing I started with classes like group cycling and aqua that required less precise cueing. Then I moved into group strength which required more cueing expertise, then finally a 32 count class. By stepping through the process I learned the skills I needed in a progression so they weren't too overwhelming.
You can also take a class on how to teach step. Try checking out the conventions that come to different areas, Empower, SCW, etc. I know NETA teaches a class in how to teach step.
The real bottom line is, though, practice, practice, practice. Before I taught my first 32 count class I went into the studio I would be teaching the class 6 days a week, put on the mic, and taught my entire class. That way I felt confident in my ability to teach the class, cue on time, think one thing while my body did something else, breathe, teach both mirroring and with my back to the class, etc.
If you aren't a certified instructor yet check out the various certifying bodies and see how they fit with you. With most of them you can either just take the test or take a class and the test. I would recommend taking the class, the instructor can give you a million tips that will make it much easier.
Shayne