If you already know how to install custom skins then you can skip this instructions.

>Unrar the .rar file
>place (or copy) the files you just extracted in:

C:\Program Files\Steam\SteamApps\''YOURSTEAMACCOUNTNAME''\team fortress 2\tf\materials\models\weapons\c_items

You will have to make some folders such as ''c_item' or ''weapons'' if you dont have them.
In ''YOURSTEAMACCOUNTNAME'' goes your, well, steam account name...