*notes to install*
Take the file and unzip then take the tf folder and put it in steam/steamapps/(yourname)/teamfortress2/ 
It will aks if you want to intergrate the files, accept then open the game to see the hat.

If you don't own the hat but still want to see the model it can be viewed by going to the in game store and finding the magnificent mongolian and clicking the preview button and then checking the 3d model option.