Just above on this very page you can click "Learn More" beneath the Workshop and News tabs. Doing so will take you to a Google document detailing pretty much everything there is to know about modding the game

Or you could join the game's Discord on their website. Someone there is making a fanmade modtool for the game in case you want it even easier

Thanks for this perfect answer!

Btw, where does it explain how to do alternate colors for units? (like how most Order units have blue and green versions)