There's not a lot of content ready for what you need to generate a cartridge yet, but that's okay because a validator is being written that most cartridges will want to use.

For the most part, it's no different than writing an XDoclet2 cartridge, but your metadata interface is JMI instead of QDox.

I generally keep the images from the UML spec handy, I've attached those pages as a PDF here in case you want to print them out with minimal hassle.