I do a alot of training regaurding when I step towards their haunches they move them away from me, same with shoulders, use a dressage or lunge whip and step towards their stomach, having pressure by your prescence on the ground and click until one step, then release the pressure and reward. Just to start
with my horses I make sure they have leg yield and quarters in and out before I would start teaching it and then I would begin to teach it from those basics!
Hope this helps
Start with haunches in on a diagonal line !