How do I maintain/get re-qualified as an ITRA instructor?

You must first be current in Level 3 of your discipline(s).  If you are, then to requalify as an Instructor, you just need to be assessed by a Registered Assessor for the instructor skills.  There are only 3 LO’s you have to do…teach a classroom block, teach a hands on block with a demo, and assess a mock student performing a level 1, 2,or 3 skill.  Depending on how many people you have in one location who need to requalify, it might make more sense to bring an assessor to you.    Or you can contact a Registered Assessor (you can find a list in the online directory) and see  what they have scheduled and where they will be, and you can go to their location for your requalification.