For placing the dog (meaning you are setting up the stack), it just takes a lot of time and patience getting the dog used to holding still and being handled this way. I never had much luck using treats. My dog gets too excited for them (even now at 17 months) and will twist towards my hands or pockets which just makes it worse. It's better to toss a toy out in front of the dog if you are alone, or have someone standing in front of him but not too close. What that person does depends on the dog. For a dog like mine, I just have to stand there and be 10 feet away. Anything more excites him TOO much. For less excitable dogs, the owners can be a bit closer and talk to their dog or even wave a toy to hold their attention and get the head up. When I am taking a picture I either use a remote or have someone else do the camera so I can stack the dog. Since he gets too excited by me, usually tossing his ball out in front works best to get him perked up and fixated enough to hold still. The reward is being release to fetch the ball.
When Nikon was 6 months, I used cinder blocks to help both of us learn where to place his feet.
I would suggest not using your "stay" command for this until later. The dog doesn't really understand what you want, so each time you say "stay" and he wiggles, it's ruining your stay command. Once the dog is more familiar with stacking, then you can place him and say "stay" so that you can step away, walk in a circle around him, and practice checking the teeth and testicles or feeling him all over.
For learning to "free stack" (the dog places himself), I start by working on the front end of the dog. Nikon is still learning this skill. Basically I lure him with a treat, stepping forward slowly, until I like where his front feet are placed and sort of halt him by pulling back on the lead. The oppositional reflex gets him to lean forward so that his front is correct (otherwise he, like many GSDs, has a habit of "posting" which means his front legs are not under him enough, or he is leaning back or hunching rather than being upright and alert forward). Then I check the rear and if the legs need to be adjusted, I quickly do that myself. Since Nikon is less angulated than most dogs we show against, typically I have to adjust his rear to stretch him a tad more than his natural stack.
In this stack, I stopped him out of motion, then adjusted the rear, tossed a toy out front, and snapped the pic.