GZDOOM :
1-Get Skulltag Emulation : Forum.zdoom.org
2-Make sure you have the last build of GZDOOM (from drd team's website)
3-Select Swan_Fox_V2_5.wad and skulltagemulationv2.pk3
4-Drag and drop skulltagemulationv2.pk3 first
5-Enjoy
Zandronum
1-Get Skulltag content : Zandronum.com
2-Select Swan_Fox_V2_5.wad and skulltag_content2.1a.pk3
3-Drag and drop skulltag_content2.1a.pk3 first
4-Enjoy