What happens if you overfill the oil on my Honda shadow?

Simple, just drain some out. You don't want to drive it that way. could blow some seals.

If you want to drain some oil off take a hose and put it in the fill hole. Then put a rag around the hole to keep oil off the motor. Then put your finger over the end of the hose and pull it out and drain hose in to a pail. Repeat until you get to the right level.