So many people have been asking this question that I'm making this guide. You've just downloaded a mod, and, chances are, you have no idea what to do with it. First of all, the author of the mod most likely packaged it as a ZIP file. This means that you need an unarchiving software (such as 7-zip) to get the folder which contains the mod. Once you have the folder which contains the mod, navigate to your C:\Program Files\Firaxis Games\Sid Meier's Civilization 4\Mods directory. If you are on Warlords, go to C:\Program Files\Firaxis Games\Sid Meier's Civilization 4\Warlords\Mods. If you installed to a non-standard directory, navigate to the corresponding folders. Extract the folder from the ZIP, or copy the already extracted folder into the Mods directory. Run Civilization IV. Go to Advanced-Load Mod, and select the mod that you copied. You can now play with the mod. If you want to load the mod on startup, go to the C:\Program Files\Firaxis Games\Sid Meier's Civilization 4\_Civ4Config shortcut, and, in the file, change the "Mod = 0" line to "Mod = Mods\YourMod". Warlords users should do the same for the Warlords subdirectory of the above path.