I teach a one rein stop on every horse I work with or own. You will have to teach her to laterally bend/flex first, and give to that one rein pressure from a standstill, and make sure she gives consistently before moving onto teaching the ORS.
I teach it 1) by sitting deep in my seat first and saying "whoa" 2) giving the horse a few seconds/strides to respond 3)then taking up one rein and pulling it to the side, and waiting for him to come to a halt and give to that pressure. I like to do some lateral bending each side, once he does stop as well. I get him solid in a walk, then trot, then canter...only advance as you see he is 'getting it'.
The reason I do it in the 'order' I do it in, is so that eventually the horse starts responding to "just" my seat and whoa vocal cue (I will usually eventually drop the vocal cue, once the horse is solid on the seat cue). The pattern will usually be the horse circling several times (this is okay, let him figure it out), before coming to a stop, and giving laterally. As he gets the concept he will stop circling so much, and will start stopping when you pick up the rein (I still flex both sides once he stops), And eventually he will start figuring it out, that he can stop to your seat, thus you no longer need to pick up the rein...but because he has 'learned' what it means, it will stick with him, and if you ever need it, that 'emergency' brake is always there.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."