Minimal domain for sphinx

I would like to use Sphinx for my own project and for that I want to create a new sphinx domain. But I have serious trouble getting started. My minimal sphinx domain would allow only a single instruction called 'command':

.. xx:command:: Somecommand

I can't find any introduction to writing sphinx domains "on the internet", only some ready to run domains for ruby, erlang etc. Can someone advise me on how to get started?

Checkout the reStructuredText domain in the Sphinx source code, it's fairly simple and should get you started. You may also find some good examples in the sphinx-contrib source.