Mod Folder/File Structure
An ExampleMod folder is created in the Mods folder for you the first time you run the game to show the expected folder structure of a mod.
Each of these folders corresponds to a folder in the game's file structure (steamapps\common\Galactic Civilizations III\data or steamapps\common\Galactic Civilizations III\DLC\EXP2_Crusade). Files that a mod replaces or that add content to a base file are placed in the folder with the same name as that file in the game's file structure. For example, ImprovementDefs.xml (which defines all the improvements that may be built on a planet) is found in the Game folder in Steam file structure, so any mod that changes it or adds new improvements to it would be in the Game folder of the mod's folder).
Installing a mod
Unzip the mod if it is compressed and place it in the Mods folder. You must start a new game to see a mod's effects. When a new game is started GC3 reads all the XML, processes it, and stores it in your saved game. Thus any changes to the XML are not seen until a new game is started.
Where to find mods
Many mods have posts describing them in the GC3 Modding section of the GC3 forums. Some of these can be downloaded from links in the posts, others have external links for downloads.
Some popular mods
This is not a complete list ...