It requires the top-secret decoder ring...
Seriously, contact the CDI (Dave Gardner and Keith Verlaque this year at
CDI@PCASDR.org).
There is a written exam on the basics of safety and car physics, etc., (emphasis on "basics"). Then there are a couple of "check rides", where the CDIs see how you communicate with students, especially really new/bad students (which the CDIs are remarkably adept at simulating
).
And voila, after the secret ceremony with the swords and flame (oops, wrong movie), you become anointed as an instructor.
We can never have enough qualified instructors, so please contact the CDIs if you are interested.