Basicly, all the stuff about the game are put into files in the main folder called pak0.pk(example file). if you wanted to mod the game you'd have to edit the info in these files. for skins and other mods requireing textures, you would have to use adobe photo shop to edit or make new textures. Hope this helps.
Sorry i haven't posted in a while, i am trying to make a sky mod for MOHAA stalingrad net demo. I found this guys mod and it was of a sky and i asked if i could make changes for personal use and all i need to know is how do i make all of the pictures and stuff into 1 file?