First, I teach my dogs to back up so that both their hind feet are touching something flat on the ground. Since it will not see what it is backing onto, the dog will need to be able to feel a distinct difference between a correct position and one that is missing the mark. Therefore, it is important that there be a marked contrast in texture between the target where they will be placing their hind feet and the surrounding area. Since I'm training on carpet, I use a flat piece of cardboard as my "target". If I were training on a wood, linoleum, or some other smooth surface, I would probably use some kind of non-slip mat as my target. With my use of the cardboard here, ideally I would affix one side with a non-slip surface in order to stabilize it on the carpet so it wouldn't slide around too much. I haven't done so here and you can see it sliding around - especially with Poppy as it goes shooting away from her at the end.
Here is Ollie repeatedly backing onto the cardboard:
My dogs are already used to playing games where they need to move and place different legs in certain positions, so they know to reach around behind them to find things without me giving them any kind of verbal or visual cues. However, if your dog is not already used to playing body awareness shaping games, you may need to lure them into the correct position the first few times. To do that, have the dog stand directly in front of you with the target lying on the ground just behind the dog. Step toward your dog so that it moves backwards. As soon as BOTH back feet are on the target, click and reward. Repeat this several times. Once your dog seems to "get it" stop stepping toward them and see if they will step back on their own. Click and reward all successes. Once you have had several successful sessions, you can start to build distance so that they need to eventually take 3 or 4 steps back before their hind feet hit the target. Build this distance gradually over a few sessions.
Here is Poppy backing onto the cardboard. Generally, her preferred method of succeeding in this game is to cheat by approaching the target head on and then doing what I call a "swimmers turn maneuver" - kind of a pivoting hand stand where she kicks up her hind legs and swings her rear end around into the correct position. Here, while the target is flat, she is not quite so dramatic, but she does approach the cardboard once head on and then turns around into the correct position. I do not reward her for that.
Once your dog is consistently backing onto the flat target, you can start to add some height. Here I use a low box turned upside-down. Here is Ollie:
And here is Poppy. Notice, once again, that she tries the head-on approach a couple of times and doesn't get rewarded for it.
I did not feel the need to add several videos of the next steps, which basically involve increasing the height of the target. I use gradually increasingly sized boxes. Whatever you use, just make sure it's relatively stable so that it won't move around or be knocked over while your dog pokes his back feet around and hoists himself up.
Eventually you can add a verbal cue or hand signal to tell your dog back. Trick taught. Now go crazy entertaining your friends by having your dog walk backwards up a variety of objects. My dogs will walk their hind legs up just about anything - including people. Here they both are using my standard go to obstacle for this - the sofa:
Have fun & happy training!