Well its rather simple, in the install instructions, it says to place it into your skin folder in "'\Program Files\Steam\skins'"
All you need to do is open the "NG_Steam_Skin.rar" and open the folder "NG_Steam_Skin" then you should see a folder called "NoobGalore." From there drag that folder into your skin folder for steam.
Then go into steam and change your skin. Hopefully this helps ya out.
Also I've been using this for quite some time, and just love it!