It is important to always remember to keep the distance you move away from the dog very short and also the duration you are away from the dog very short, to begin with. We can build on this at a later date, but only if we keep things short and simple to start with.
The hand signal for Stay, is a flat hand, closed fingers together and palm facing the dog. Ask the dog to sit and immediately, hide the hand holding the treats behind the Stay hand signal and rock backwards quickly, returning before the dog moves out of position and treat the dog.
Now repeat, rocking to the left, return and treat the dog. Now repeat rocking to the right and return to treat the dog.