Hello there!
It’s not really my topic, I’m too much of a realist for that and have created quite a few examples of in-depth tagging and special things I’ve come across in my immediate neighbourhood through intensive trial and error and self-checking.
But I have seen a bit of reading material for fictitious mapping over the time:
The first address is certainly: https://www.opengeofiction.net
Further tips and hints could be found here:
Never finished, but a clue: User:Ika-chan!/Fantasy maps with OSM software - OpenStreetMap Wiki
For advanced users a possibility: Sandbox for editing - OpenStreetMap Wiki
Various other possible additions to the topic: Develop - OpenStreetMap Wiki
How not to do it in any case was discussed here:
F wie Fake-Land... I think it’s done funny so far though and admire the energy the mapper has put into it, unfortunately in the wrong place.
Good luck with your project!