How to teach your dog to hold a position until released.
Stay teaches your dog to hold a position until you say so. A release cue, such as "free" or "OK," signals when the behavior is over. Without one, your dog has no clear marker for when they can move. This is as much a part of the behavior as the stay itself.
Use small treats your dog finds rewarding. A verbal marker like "yes" or a clicker helps you time rewards to the hold. Work in a quiet spot with few distractions. Begin each repetition from a sit. Keep sessions to 5 minutes or fewer.
Ask your dog to sit, then wait one or two seconds before rewarding. Don't say "stay" yet. You're teaching them that holding position pays off before adding the word. Once they hold the pause reliably, stretch it to 3-5 seconds, then a little longer.
Once your dog holds a few seconds consistently, say "stay" after they sit. Wait, then reward while they're still in position. Follow that with your release word and let them move. Rewarding before releasing teaches them to wait for the word, not the treat.
Step back once, then return to your dog and reward before releasing. Walking back to reward teaches your dog that the stay continues until you say so. Once they hold with you one step away, increase the distance gradually.
Stay falls apart most often when duration and distance increase at the same time. Work on one at a time. If your dog breaks, shorten the next rep. A shorter rep that succeeds is more useful than a longer one that fails.
A reliable stay takes many short sessions. Most dogs hold a few seconds with you nearby long before they're ready to hold at a distance. Progress at that pace, and the behavior will hold up when you actually need it. One of the most useful places to use it is the front door; see how to stop a puppy running out the door.