After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
Drop that folder into the mods folder which is supposed to be in the game's installation folder (don't know where that is? click here)
UNINSTALL AND RESTART THE GAME TO LOAD YOUR OLD SAVES!!!
The mod will use save and backup slot '65' to save your progress, this means that you have to start over. The mod will also save under 'local_hdd' instead of 'steam_cloud', but i still advise you to backup your main saves manually.
REMOVE PREVIOUS VERSIONS WHEN INSTALLING UPDATED VERSION!
I have no experience in auto updates so you have to manually update this mod.
(THIS IS NO LONGER THE CASE ON VERSION 14 AND ABOVE, BUT ON OLDER VERSIONS YOU HAVE TO UPDATE MANUALLY)