1.) Navigate to your steam directory under which you've installed Fortress Forever to. (The directory should look something similiar to this: "C:\Program Files (x86)\Steam\steamapps\common\Fortress Forever\FortressForever"
2.) Once you've located this directory, go ahead and locate the "models" folder within. "..\common\Fortress Forever\FortressForever\models\weapons"
3.) Now that you're in the "...\models\weapons" folder from the previous step, all that's left to do is drag and drop the custom files downloaded inside this .ZIP and allow them to overwrite any existing files.
4.) Profit! You're done, it's time to play the game. :)