the way my wife did it to our dog. try teaching back at first while they are in a heel position and walk backwards with them on leash while the dog is against a wall. they'll be sandwiched between you and the wall and will have to back straight up. treat reward as soon as they start to back up. make them go a little further every time. when they are backing up that way, change so you're facing the dog but walk forward with them while saying back. lure them with a treat over their head if needed. again treat when they take the smallest step back and make them go a little further every time. after that your dog should just be able to back up with you standing in place. could take a few days to get it good.