Alway use small treats when training dogs. You don't want to fill them up because dogs can become disinterested when they're full, which makes training difficult.

I always grab a handful so I can get the treats to my dog as fast as possible. Dogs only have 3 seconds to associate the treat with the action they did correctly.

Get the dog's attention by letting them smell the treat and maybe even get a lick of it.

I use a flat hand as my signal, so when I hold the treat I try to have it look as similar as possible. Push the treat toward the dog's face at nose level.

Once the bottom touches the floor, you can give the dog the treat!

The idea is that you're keeping the treat close to the dog's head, the dog's neck is straining to grab the treat, and the butt goes down automatically.

Always release your dog letting them know they're done with that action and you aren't going to ask them anything else in that session. I use the word OK and pay my leg.

Up the ante when they're good at sitting with the treat, to trying to just use your hand signal with no treat in hand. Still give treats, just store them in your other hand.