Crawl is signed by taking your hands and placing them one in front of the other. The movement should look like the hand motions a baby makes when crawling.
An alternative ASL sign for crawl is to get your non-dominant hand, flat and facing down like a surface. Then get your dominant hand, horizontally forming the number '2' or the 'V' shape lying on its side, and curving the two fingers (pointer and middle) as your hand crawls along the floor (that is your non-dominant hand), from the wrist to the finger tips.
Crawl and walk are good words to contrast against each other. You can make a game of crawling, then walking, using the respective signs as the signal to switch.