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.
I used essentially the same method as Jessica's trainer friend.
I would place Abby in a down and show her the treat. I used peanut butter on a spoon or bits of plain vanilla ice cream on a spoon because my dog really likes those. Then I would slowly pull the treat away in front of her and say "crawl".
One issue we did run into was that her behind tends to pop up when she is crawling, especially if she's really eager to get to the treat. I have an old riding crop that I use as a pointer and for giving directions when I train with her, so I would touch her back above her behind to remind her to put her butt down, and that worked well.