Does she know how to leg yield under saddle? And can you get her to yield her hind quarters and shoulders from the ground? Until you can get these with no problems, don't worry about turn on the haunches.
They way I teach turn on the haunches from the ground, is actually the same aids as what I'd give under saddle, minus the weight/seat. Put the horse in a bridle, and put the reins over its head like you would when you ride. Practice having the horse walking forward and back up while keeping your right hand holding both reins just above the wither. You don't need a corner to teach turn on the haunches, just a straight fence of some description if you would like to add a bit of support for the horse.
(Imagine you want you horse to yield to the right for this explanation) Face your horse, standing just behind the shoulder, reins in left hand with the right rein slightly shorter. Ask the horse to flex a little to the right. Apply pressure with your finger tips behind the shoulder/on the girth. It may also help to have a dressage whip handy to tap the hind legs forward if need be. If you are asking correctly, and the horse understands it, the left fore leg will cross in front of the right fore leg. The left hind leg should also take a small step forward. Turn on the haunches, as all movements are, is a forward movement. There is some forward momentum as you want to left for and hind to step forward and across rather than backwards, which often occurs when you start to block to forehand.
Under saddle, I will weight my inside (right) stirrup, move my outside (left) leg back behind the girth, take my inside (right) shoulder back a little and ask the horse to flex to the inside (right) with my inside (right) rein. Ask the horse to move across with your outside (left) leg, while supporting the bend and keeping to forward momentum with your inside (right) leg.
Be happy with one or two steps, then walk off onto a straight line. Don't over do turn on the haunches training as it does put a lot of strain on the horse's joint, particularly in the hind quarters if the horse is not accustomed to having to 'sit' yet.