To install, simply navigate to the "Tools" section of your Steam game library and find "Sid Meier's Civilization V SDK" in the list. Install that. Create stuff. Profit*.
2K Games has also set up a wiki for users to populate with all types of tips and tricks when it comes to modding the game. Mods can be viewed and managed from within Civ V for easier organization.
*Profit not guaranteed or likely.
Thank goodness. It was hard enough to cheese achievements as it was.