First, check out the mountains in your area and see who is hiring. A typical resort will put people through both off and on snow hiring clinics which help to teach how to teach snowboarding. From this pool of people they will pick instructors to hire. It does help to be able to ski as well.
If you make it past this point there is typically training provided by the school to improve your skills and get ready for exams with AASI. In AASI after becoming a member there are three levels of certification. I, II, and III followed up by variety of accreditations in children's, freestyle, and TA(trainer's accreditation).
The training for the cert I is pretty short and straight forward; as the clinics and exam are usually wrapped up into one. For higher levels of cert it is necessary to do prep clinics for the exam.
You might be able to obtain the first level of certification with a prep clinic. To be able to do that without working for a ski school you would need to contact the Eastern region AASI to sign-up and to see if they would let you. One thing to note. If it doesn't workout don't try to poach (i.e. teach at a mountain you don't work at for money). It's cool to show friends how to board. However, when it crosses the line into making money, you need to understand that you are taking money away from the resort, and its instructors. You are also screwing with liability of the mountain, and exposing yourself legally. And in this day age the legal ramifications can be down right scary.
I wish you luck and hope it all works out.
Last edited by gjsnowboarder; 12-11-2012 at 10:53 AM.
Reason: spelling. etc