I dont actually use the 'stay' command with Betty.
I'd put her in a sit, down or stand and make sure she knows these words and the hand signals to go with them first.
Then I'd repeat the command and slowly step backwards, repeating the word and doing the hand movement each time I step further away.
If she moves go staight to her and put her back in the same place, dont say anything else, just the command(sit, down, stand) and start again.
Build it up, going further each time, then try walking back towards her, again repeating the command, and straight past her to the other side.
I love practising this with Betty, I'll try and 'catch her out' by running towrds her, then straight past, but putting in a command, to test her
sad arent I