I would begin studying for the FOI and FAI written exams. That will refresh your memory and help get the idea of what to learn to effectively teach. Get those out of the way before go and spend 5 or 6K at a flight school. Might also go to the local FBO and get checked out in a trainer to get rid of the rust. Might be shocked by the inflated costs, probably double hour rate in past 20 years.