Your teaching qualification would be your first step and then you have to either write your own courses, get them accredited etc (or "buy" a course with GTI) depending on your ethics.
There is advantages and disadvantages to both, if you do it all yourself you are liable for the courses, kits, anything to do with the students, products you use etc but it gives you the independence of teaching the courses exactly how you want (as long as the insurance backs you up).
If you teach for a brand you have got the backing of the company, the team, they will deal with any enquiries, product and kit supply, they ensure that everything is insured and complies with legislation and they also help with marketing etc.
I would say both options are equally popular and its a bit personal preference of what you like and how much time you have for all the organisation and admin stuff, advertising etc x