Step 1: Download Doom Legacy and the Doom2.wad v1.9.
Step 2: Install Legacy doom and extract the doom.rar (Using WinRar) to the Legacy directory.
Step 3: Run launcher.exe in the Legacy directory and click the "Game Mode" button.
Step 4: Enter the path to your legacy.exe file in the top box and enter the path to your doom2.wad file in the box second from the top. Type "Default" in the dropdown box in the lower right. Click OK.
Step 5: Click "Setup" and select your preferred display mode from the drop-down list. I strongly suggest "OpenGL". Click OK
Step 6: Click "Launch" to start the game!
Step 7: Set up the controls and screen resolution in the "Options" menu (Duh).
If you get a problem finding "doomargs.tmp", just create an empty file with that name. All should be rosy.
Step 1: Select Multiplayer/Connect Server. Change the "Search On" value to "Internet" and select "Refresh".
Step 2: If there is a game running, a server will appear in the list, select it to join the game. Sometimes (Read: often) the master server is down, this means you will have to get the IP of the person that is hosting and enter this into the "Connect to specified server" box in the multiplayer menu from the main launcher program. Hit OK to join the server on the selected IP.
Step 3: Shoot monsters!
Step 1: Run launcher.exe and click "Launch".
Step 2: Select Multiplayer/Start Server.
Step 3: Set "Map" and "Skill" to preferred settings.
Step 4: Set "Monsters" to "Yes", "Deathmatch Type" to "Coop" and "Internet Server" to "Yes".
Step 5: Enter a relevant server name in the box at the bottom.
Step 6: Extra settings can be changed in the Multiplayer/Options menu before the game has begun or while the server is running.
Step 7: Select "Start".
Step 8: Shoot monsters!