Open it with any text editor (wordpad is okay I think) and start modding.
(v 1)\XML\' folder you'll find a file called 'CityStates.xml' Once installed, in the '.\My Documents\My Games\Sid Meier's Civilization 5\MODS\Y.A.G.E.M. I'll describe the ones in Yagem, as I now it better Well, you can find some easily in the mod browser, and in the first post both 'Yagem' and 'Not Another City States Mod' contains some CS.
