Well, looking at your sig line, I'd guess you're a "gun guy"
as am I... You may also be a good shot, as am I. I was able to put myself through Glock Armorer's School, with a letter from my boss for the registration, but I took the day off and paid the fee myself... None of this will get you to Instructor's school at my agency. In fact, my agency didn't have ANY instructors. We always went to the range with the Sheriff's Dept. and they did our training and our quals. What it took for me to go was the following:
1) Accomplished something at my job that looked good (had nothing to do with firearms).
2) Overcame the "he-just-wants-to-go-to-FI-School-to-make-himself-better/look-good/have-fun" impression that any "gun guy" gets labelled with, by expressing in my letter that my interest has changed from my own shooting / competitiveness to a desire to help others become better shooters.
3) I had to show why it was useful to our dept. to have an internal FI rather than just stick with the Sheriff's Dept.'s program.
4) Our agency weapons/use of force policy hadn't been re-written in 11 years, and needs an update severely, so I pointed out that attending class would give some insight to help with that task as well.
5) My direct supervisor interceded with the top boss to show that it wasn't just my idea (again, important to overcome the "he-just-wants-this-for-his-own-self-enjoyment" attitude).
6) One of my main points and desires was to do some more-advanced "combat" training, as opposed to the training we do now.