Get your dog to sit and give her/him praise. ( sorry Shelby doesn't like cameras)
Get her/him to lie down
Make sure to say crawl so they can get used to it
And every time try to make it longer and longer
Then try the Invisible Treat method. You act like you have a treat and at the end give them praise. You won't always have a treat with you
Keep doing it until you don't have to have a treat or act like you have one. And say crawl. ALWAYS give them praise no matter what!