To deathmatch on line the easiest way is to make sure you have the correct versions of Doom, the most important being Doom2 v1.9
You now need:Any/Either/All of the following in no specific order:
CSDoom
ZDaemon
IDE (internet doom explorer)
Doom Connector
Skulltag
Khan seems to be the option of choice for people wanting to use original Doom2.exe
Run and set paths, then hey presto , online Dooming.