STEP 1. - Own the game (i think you can install mods with a pirated version of the game but shhhhh)

STeP 2. - Install the game

StEP 3. - Go to the installation directory of your game. (in steam right click the game, properties, local files, and click Browse...)

SteP 5. Transfer the mod's data file to the installation directory and replace it.

Step 7. Celebrate