You use a concept called shaping. This is best done with clicker training in which you mark successive approximations of the goal behavior with an audible sound made by a clicker or other device, and follow each click with a reward. For example, you would click when the dog puts one paw on the skateboard, then two and so on, until the dog is finally moving on the skateboard. Each click is followed by a food treat as a reward, which serves to reinforce the behavior. The clicker is a communication tool used during training. Once the dog learns how to use the skateboard, the clicker is no longer needed.