the mods folder is in \MyDocuments\My Games\GalCiv3\Mods\ just add a folder and put a Game and a Text folder in it, copy xml files from the steam install directory usually \SteamLibrary\SteamApps\common\Galactic Civilizations III\data\ and edit them to suit your needs, you can either use the full replacement method or the append method.
If you've ever worked with xml mod files you should find it fairly straightforward.
Here is the link for the modding post the dev's put up.
https://forums.galciv3.com/461766/page/1/
