Have someone behind her with a line and prong collar. You give the back cue and they start back pressure and anytime she gives an inch back you mark reward and release pressure. You might need to keep the reward out in front of her at first to keep her from turning around to walk to the person putting pressure on her.
Anyway steadily fade the reward out as you're upping the criteria of getting more backward steps and you should be off to the races. You'll of course need to fade the back pressure too. It shouldn't take more than 1-2 sessions. You just need to show the right picture.