At five months she probably won't really get it yet.
It goes against everything a puppy of that age thinks should be happening: I want to run, I'll run, my human moves away from me I'll follow and run after them.
It just needs repetition, repetition, repetition and treats, treats, treats.
After a lot of training my dog to stay we eventually got to the stage where we would arrive at the park, she would sit down and wait for me to walk some way away from her, I'd then call her and she would come running for her treat.
She clearly thought she'd trained me well, and she had, but she was a great deal older than five months when she started doing that.
Just keep working at it and give it time. She will get it.