Drag & Drop the included 'Client' folder into your TERA installation directory. You'll know you're in the right place when you see another 'Client' folder too. It's okay to merge the folders, as this mod will not overwrite any of your files. No need to worry about backing up! It simply adds it to a new directory entirely, and your game will read it, leaving your original safe and unharmed. You only need to go to 'TERA\Client\S1Game\CookedPC\Art_Data\_Packages\CH\Accessory', and delete the Acc_177.gpk should you want to uninstall.