The key thing to remember is that it is a shoulders in, not a haunches out.
The hindlegs continue tracking straight down the wall as they would were you just trotting down the wall normally. The mistake I see most often is the rider pushes the horse too far over the outside aids and the haunches swing out to keep the horse's balance. The purpose of the shoulder in is to ride the horse up to the inside aids, not push the horse over from them.
Focus on that outside hind and keeping it tracking straight forward, the hind legs should not cross. Then bring the shoulder in with your outside knee, and sometimes an opening inside rein (as a teaching aid). The inside leg stays pressed forward and waits for the outside aids to push the horse up to it to create bend. At first, dont try for 3 tracks, begin with a shoulder fore, in which the outside fore is between the hindlegs, so the horse is on 4 tracks.
Good luck! The shoulder in is one of the most difficult movements to ride correctly.
They say money doesn't buy happiness -- well happiness doesn't buy horses!