Open the game, click the "Mods" button in the main menu, and click the "Help" button in the top right. This will provide a fairly comprehensive guide for all things mod related, including how to install them.
Note: when finding a mod you want to use, check to see if it has been updated since the beginning of this year or that it matches your game version. Several smaller mods, like the improved random portraits mod, work despite updates to the game, but anything related to the sex system will not be so lucky. Also, many mods come with install instructions, so keep an eye out for those.