it's easiest if you teach it in hand first, so she knows what "back" means, and then teach it with someone on the ground to gently push backwards on her chest as you apply the aids, at least the first few times until she knows what you're on about. it goes without saying that you need to be able to get a good walk and a good, perfectly square, balanced halt every time for a while, so you're starting from an ideal halt, to make it easier for her to rein back. the aids which i was taught are putting both legs behind the girth, quite far back, and being light with the mouth... never pull back. once they realise that that is what both legs swung back behind the girth means, you won't need to use the reins other than very very lightly to stop her from walking on. be happy with a stride or two at first, lots of praise, and then gradually add steps. walk forward immediately, as soon as she's gone back, to keep her thinking forwards all the time... very important. good luck!