Have a guide for writing WS in Nuxeo using Metro

The goal is to have Metro the unique provider for WS framework in Nuxeo 5. The users would have to know how to write a custom WS following the Metro.
The points to be touched:
I want to expose a WS API on top of Nuxeo. Ideally I just want to write a simple Bean and let the magic happen.
=> how to do that ?
I will also need to have used authenticated and possibly to obtain a CoreSession
=> how to do that ?