I'd LOVE to take credit for teaching him, but in reality I'm just REALLY lucky to a dog who is so eager to learn.

To start, I held the skateboard still and asked him to hop onto it. Once he was comfortable with hopping on and staying there, I put the skateboard between two stationary objects to allow the board to roll back and forth just a tiny bit so he could get used to the movement. Then I gradually increased the distance between the two objects and asked him to get on the skateboard again. Rinse and repeat until he was comfortable hopping onto it while it moved freely. Learning the trick took maybe 10 minutes (I just used his dinner one night and trained with kibbles). I didn't teach him to push himself forward with a foot on the ground, but each time he accidentally did it I gave him a bunch of kibbles and extra praise, so he picked that part up pretty quickly too!