You need a copy of Doom 2 and Skulltag. Install Doom2 and you should be able to run Skulltag (make sure both doom2.wad and Skulltag are in the same directory). Now download Ghouls vs Humans. You should be able to run it by dragging the .pk3 files into it but you can also use a wad launcher. If you want to join a server, or host a server you have to use IDSE which comes with Skulltag. Any problems, join the Skulltag Forums and ask away.