Lots of lunge line, stirrupless, and bareback work to develop a strong seat and leg. As for position, have the person stand on the ground in the position they should be in while in the saddle. They can see how if they have their legs too far in front of them it makes them topple over backwards. Where if your body is balanced you can maintain position, your body should be this balanced in the saddle. Do 2pt and standing work. Have the person stand up in the saddle, crotch over the pommel. They need to be balanced over their leg, not resting on the pommel. Maintaining this will build leg strength. When the person sits down, don't let them move their leg forward and it will help them get their leg under them. Also 2pt, sitting down without moving the leg. Build up from maintaining the standing or 2 pt position while walking, with longer periods, to maintaining while trotting. With 2 pt build up to maintaining at the canter. Posting without stirrups is really good, but will be killer at first. Also, funky posting. Stand for 2 beats, sit 1 beat, stand for 2 beats and so on. However of course they need to first build up to be able to post at all. This was really brief but I hope I got the point across

Before you criticize someone walk a mile in their shoes. That way you'll be a mile from them, and you'll have their shoes.