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1°) Download craft the world mods somewhere (Best is SteamWorkshop with steamcmd but i don't know how to do this, but there is many topics about this on web)
2°) Unzip the mod file
3°) Copy the mod folder in "C:\Program Files (x86)\GOG Galaxy\Games\Craft The World\mods" (Create "mods" directory if doesn't exist)
4°) After restarting the game, ods will be anabled in mods menu in game.

I hope that will help you.