***** The process looks a little long, at ten steps, but it only takes about 5 minutes in total on a 1.5Mbps Internet connection, and half of those 5 minutes is waiting for the download. *****

1) Download DoomsDay Engine from here: http://dengine.net/windows

2) Install DoomsDay Engine by running: doomsday_1.12.0.exe (for example).
2a) I'd recommend just installing it to the default location: C:\Program Files (x86)\Doomsday (for Windows 64-bit).
2b) Perform a Full installation, it's only 45.8 MB

3) Once the installation has finished, opt in to Launch the DoomsDay Launcher

4) Select your language.

5) Select which 'Game Profiles' you want to set-up. Only select games you have installed already at this point, or games already downloaded via Steam.

6) It will prompt you to find the main data file of the game, such as DOOM1.WAD (Shareware) or DOOM.WAD (The various Dooms), or DOOM2.WAD (for Doom II).
6a) You can just browse to your Steam folder here: It should be something like [drive]:\Steam\SteamApps\common\Ultimate Doom\base\. or [drive]:\Program Files (x86)\Steam\SteamApps\common\Ultimate Doom\base\. for The Ultimate Doom.
6b) You could also just type "*.WAD" in the Search box in the Choose Game Data File window, with the drive or folder you have Steam installed in. It will search all sub-folders this way.

7) Click next on 'My Addon Folders', as you don't need any to get going.

8) You can opt to have the front end close when the game is launched, this is only really useful on older systems to save resources.

9) Click [Finish].

10) Double click on any of the games you've setup via the Game Profiles above to launch the DoomsDay Engine Source Port of that version of Doom. (Such as Doom, Shareware Doom, Ultimate Doom, etc.)

Once you're in Game, because it's a Source Port, there is a join in progress, join multiplayer game option to just let you jump into the action.

Message me, and I'll start a server.