I am still a novice. Realistically I SUCK at stacking my bitch. Luckily she can free stack really nicely from gaiting. But things I learned while learning is this:
-control the head, you control the dog. I struggle with this, but I see pro handlers do this ALL the time and it's literally like magic. They have the head and the dog just STAYS there. So when I control the head, I'm using a slip chain, right under the jaw line, and I hook a finger through the loop at the back of the head so I can keep it taut. The leash is bundled in the palm of that hand so I'm using only my thumb and pointer finger to keep the chain secure and to keep the dog's head under control.
-to reposition the hind legs, it is infinitely easier to literally scoop the dog up by their bottom and let it drop naturally. I place my left hand between her legs, lift up so her entire hind end is OFF the ground. And bring it back down. It allows her body to reset naturally and 95% of the time results in proper alignment.
-You might not be able to SEE what it looks like, so it's easier to go off feel. You know the elbows should be under the shoulders, so when you are positioning the front legs, feel the shoulder, and picture the line from the shoulder down. Then grip the elbow and align it with that line. Do the same with the hips. Knees should be under hips.
-Teach your dog "step". This will be handy as heck when you are stacking on the down and back. You gait down and present your dog to the judge. The dog should be able to free stack here but if a foot is out of alignment, you can nudge the dog into a stack by telling it to step. You can use leash pressure to communicate this to an extent. I never thought it was possible, but my own girl has done it with just leash pressure alone.
This might be a helpful resource: Conformation Stacking
I hope this helps!