We've all been very busy today!
I've been doing this too today!
I've been doing it with mine for a while, and I think its a bit of trial and error with what works for you and it takes a bit of time too.
I'd start by getting them to be able to do positions at a distance when staying still. Start where you are right in front of them and ask them to do a sit, down or stand. Take a step away and repeat, two steps away and so on. If they stop at any point go back to them and start again. I've tightened the criteria, as mine tend to do a little walk forward when I say stand and I want them to do it without moving forward.
To do what is a stop on target, which you can build up to when your timing and their response times improve, I leave them in a stay move away then call them. I found to begin with a stop command with the stay hand signal got the best results. I found they naturally went into a sit, so now I just do STOP and hand up flat and they stop. Sometimes immediately sometimes not.
Just another way round it. Find which works the best for you and your dog.
Then try the emergency stop.