I'll just tell everyone step by step how to do this just in case they missed something and they want to do this to their own skins. I will demonstrate with RF's skins he just made...
1) Open the pk3 with WinRar.
2) Go to the models/player folder and find "german_3rdss_totenkopf.tik" and "german_3rdss_totenkopf_fps.tik"
3) Drag the .tiks onto your desktop or someplace where you can rename them to a stock skin from the game.
4) Next rename "german_3rdss_totenkopf" to german_panzer_shutze
5) Then rename "german_3rdss_totenkopf_fps" to german_panzer_shutze_fps
6) Close winrar and go into a game, choose the german_panzer_shutze skin and you will see RF's 3rd SS Totenkopf Skin
*Note* You will not see the .tik extension on the file when you drag it onto your desktop, and you do not need to add the extension when you rename it.
I hope this was easily understandable and I hope that you can apply this to your own personal skins that you use to make your game a wee bit more interesting
Peace