I created this video a while back:
https://www.youtube.com/watch?v=djQh8XKRzRg not sure if you've seen it yet. It's a bit outdated but does go over some of the basics of working with designer.
I should point out that the designer, in its current form, is more of a proof-of-concept, then a full fledged tool. It's a demonstration of what is possible but it's definitely not ready for you to build your next app upon. There are members of the Polymer team who are now exploring the future of tools like designer, but that work is still in the experimental phase and I don't know if there's any ETA on when they would have something to show publicly.