[baekjae]

Hey, to teach crawl. Your pup will have to know "down" as a pre-requisite.
Anyways, once you tell your dog to "down", you can use a treat to lure him forward from the down position. Start off slow, so lure it maybe a few inches away from his nose, and as he gets the hang of it, move the treats farther away.
If he gets UP to go foward, dont give him the treat. Instead, reset. In other words, tell him "down" again, and try again until he's crawling to go forward.
After he can crawl with the treats at a good distance away from him, you can start adding the queue word "crawl" and start associating the word to the action.