Lisa, FWIW, I train this initially by sending the dog to the target, then running after them and giving the "sit" command while right behind the dog. (it's a nice workout for me.
) It has to be repeated a lot
. A whole lot. Over and over. I do it in my living room a lot, so I don't have to run as far.
What I found with Elmo was that eventually, when I'd done this enough, he started getting the idea to turn & sit, and I could run a little slower, so I was a step or 2 behind when I gave the sit command. Gradually, I was able to increase it. Once he was clear on the idea, I was able to increase faster. It took a long time to get to the initial phase of my hanging back a bit
The biggest bog-down I ran into was that I was letting him get his treat off the target, then asking for the sit, and marking and rewarding that. I had to start giving him his marker word when he reached the target, before he got his treat, and then again when he did the turn and sit. He then understood that he was supposed to do both things; go to the target, then turn & sit. He has a pretty nice go-out, though I'm a slacker about practicing so it's not as reliable as it could be. (it's a hard command to keep up on)
With Tully, I'm being quite a slacker, so I haven't gotten to the point where I can start hanging back yet.