A trainer friend who teaches this in her Canine Acting/Tricks class has the dog lie down, then holds a treat near the floor, about as far as the dog's neck can stretch out and grab. She s-lo-w-l-y pulls the treat away while saying "Crawl", encouraging the dog to creep a little to get the treat.
She repeats this over and over, increasing the distance a little bit each time. Good luck! It's a very cute trick.