Multiplayer:

-open ZDL
-goto settings
-source ports/engines : add the gzdoom exe of the file (not others gzdoom)
-iwads : add DOOM2.wad (also works with tnt.wad or plutonia.wad)
-return to main : external wads 'NOT' add nothing.
-map: load map03 (map01 and 02 is only for single player).
-skill: hard.
-game mode: coop
-for the hoster: put the number of players and later press launch button (remember, the match only start if all the players enter the server, that mean if you put 4 players, the match not start with 2 or 3 players)
-for the client: put the ip address of the hoster and put on players 'joining', later press launch button.
when you enter to the server you will see a arcade machine, enter on the gray floor, the match not start if the other player not enter to the gray floor too.