Are you already a certified group exercise instructor? If you’re not already a certified group-ex instructor, that would be your first step.
There are several paths that I can think of, varying in how challenging they might be to accomplish.
1) AFAA does live trainings to prepare instructors to take the AFAA exam. That’s one way to train instructors.
2) Some clubs and chains have in-house trainings, although they’re getting more rare. For example, the YMCA used to have its coordinators teach the Fitness Leader and Fitness Instructor programs.
3) Design your own training program and market it to new instructors. I’ve only seen one instructor who has been successful at this. She had a 4-week program with weekly two-hour meetings to teach beginning instructors how to teach, how to cue, etc. She did NOT call it a certification. I think she called it a teaching skills workshop.
4) Apply to be a master trainer for a format that you enjoy.