I prefer not to use a separate "stay" cue. "sit" means SIT until the release. "down means DOWN until given the release cue. When you start, you reward very frequently to maintain the position. Once the cue is established it becomes your responsibility to keep the dog in position, using only rewards, until you give the release cue and toss the treat so the dog has to change positions to reach the treat, Now the dog is set for the next practice. The better you are at releasing before the dog breaks the stay the faster the training will go.