How old is the student? I would stay away from theory until they’ve had a chance to feel the instrument and feel the scales. I would start with the physical aspect first—let them get some immediate reward. So whatever the basic is on accordion—probably figuring out how to squeeze and getting a feel for different kinds of squeezes.
Music has always been more of an intuitive thing than an intellectual thing. I am a trumpet player and I’ve taught a number of trumpet students. It does help if the students are better educated, but the most important thing is establishing a good rapport with your student. If you don’t do that, they’ll quit (unless a parent is forcing them to do it).
I might listen to some accordion music—recorded and then you. Maybe after the first lesson, I’d leave the student practicing making sounds and, possibly, making rhythms. Different ways of making rhythms. I would work on sound a rhythm for a while before introducing melody. I would work on melody for a while before introducing chords.
In fact, that’s your lesson plan for the first year, probably. Don’t push them too fast. Make sure they have a real solid ability on one step before moving on to the next. No tears (that’s for the parents). They know their own limits better than we do. Keep it simple. Build slowly. Maybe after a year, you might start adding theory—but at a very very basic level. Depending on how old they are, of course.