I thought it would be easy and quick to set up a shared room resource calendar between two separate domains on O365.
It wasn't! And I couldn't find anywhere online describing how to do this seemingly straightforwards task.
Also, it's not perfect. It can take up to 4 hours for the new appointment to appear in the remote user's view of the calendar (apparently MS are working on fixing this).
So here are the steps I used:
Set up two-way organization relationship between both domains (do this in both directions):