You will then be greeted by an information. Accept the license agreement and click 'Next >'. You will get a prompt asking if you want allow the application to make changes to your device. For problems during the installation, see Troubleshooting. No longer do I have to chose between 'conquer everything with a massive army' and 'play a friendly city-building game'. For multiplayer compatible installation, see How to Install the Modpack. The best part of Barbarians Evolved is that you get a full warmonger game (and then some) while still being best buds with everyone thanks to a universal shared enemy (the Barbarian civ) and various liberation bonuses. I go even further and use stuff like the Promotions Expansion Pack mod: It's hilarious to see your early units slowly turn into unstoppable death machines with 20+ promotions thanks to the nearly limitless EXP.
After finding a file or mod that you would like to use, simply. Almost all of the files there are ZIPs, a file type that can be opened using a program like 7-Zip, which is free. Surviving the early/mid game and then slowly liberating the world is fun as hell. Civilization V together with Brave New World is definitely a complete game that works great without a single mod. All types of mods and content can be downloaded from the Civilization IV File Database, which organizes files by type (scenario, map, mod, etc).
I will second this, Barbarians Evolved is easily my favorite Civ 5 mod, and the one I play the most.
