1. You have to be proficient at flying
2. You must have an aptitude for teaching
Having an aptitude is only a starting point, that aptitude needs to be developed into a skill before its of any use to you. I spent a number of years training people who had been selected for their apptitude, some didn't make it, some shouldn't have made it and one most unlikely student finished up as the leader of a World famous aerobatic team. Atttude is also very important. Ab-initio instruction does not require the range of experience that operational training might benefit from. The RAF used to take good students straight from training and recycle them as instructors without having any experience outside the training environment.