To teach your dog to stay, you need a little more time and patience than when you teach her to sit. This is because you have to teach the “stay” command in baby steps.
Have your dog sit in front of you or beside you (it doesn’t really matter either way). Have treats in your pocket, but NOT in your hand. Hold your hand flat in front of your pooch, palm a few inches in front of her nose. Say her name and tell her to “stay”. Hold your hand there for a count of five. Then give her her release command, treat her, and praise her. Then repeat. If she gets up during that five count, make her sit again and start over. Once she stays for a full count of five (count slowly. Use Mississippis if it helps), go to ten, then fifteen. Switch up the length of time she has to stay still, but the entire time you want her to stay, keep your hand out in front of her where she can easily see it. This will be her signal to keep staying.
Once she stays in front of or beside you for however long you hold your hand out, it’s time to move to the next step. Literally. Tell her to stay, give her the palm-out sign, and take two steps (small ones) away from her. If she follows you, lead her back to where she was and do it again. This may take a week or so of daily sessions, but she’ll learn that when you tell her to stay, she’s supposed to stay no matter what you do.