Goal: get your dog to walk backwards
Start with a handful of small treats. They should be small enough so that you can rapid fire feed them. Have your dog in a stand in front of you, put both of your hands together down at your dog's nose level between your legs. Slowly begin to walk into your dog and click and treat (c/t) the moment your dog takes a step back. You should be watching the back feet and clicking for thoses feet moving. As your dog gets better at it you can chain two or three steps together before clicking.
In the mean time, think about the cue and hand signal you want to use. When your dog is beginning to step backwards when you step into them you can fade the food lure and add your word. Keeping your hands in that low position will help the dog as well, just don't have any food in them. Only ask for three or four steps before clicking. As your dog gets faster and faster you can ask for more steps and move your hands to where you want them to end up.
If your dogs begins to fade to one side, step back to recenter and begin again, just ask for fewer steps. Try to always feed in a stand to prevent the falling into a sit.