The trick with teaching adult beginners is that you have to give them enough of the basics to keep them safe, but you can't drill them in the same way you do juniors or they'll rebel. The fact that they've refused to take formal lessons tells you they have zero tolerance for trotting a circle in the dust being told "Heels down, look where you're going, back straight."
So....you have to cover position, but focus on the functional aspects -- if you do it this way, you're better able to control the horse; not, it looks better or what's expected in the show ring.
As far as basic control, once you've taught them the mechanics, do lots of games...red light, green light, Mother, May I, and similiar ones, that focus on their ability to control their horses independently, rather than moving as a group. Motor vehicle metaphors work well with adults -- talk about safe following distances, how hard it is to turn without forward motion, etc.
Set very specific goals - I want you to be able to catch, groom and tack up by yourself without assistance; I want you to able to w/t/c both directions of the ring and perform simple figures without coaching from the ground, etc., to keep them motivated.