• The only thing you will need to download is Zandronum
  
• Download the Installer (Bundled with Doomseeker) version

Link for Zandronum Download: http://zandronum.com/download

• Open your Zandronum installer
  
• Chose to install the program into your Steam folder where your Ultimate DOOM is located and select the "base" file to install it into it
  [Mostly it is C:\Program Files\Steam\SteamApps\common\Ultimate Doom\base - but if you have your Steam installed in other file go there]
  
• Sellect Portable installation
  
• And then press Extract to get the installation up and running

• The last thing you need to do is remove the dosbox file and replace it with the Zandronum file
  
• Go to the folder where you installed Zandronum and look for file dosbox and delete it

• After you delete the file scroll down until you find a aplication called zandronum and rename it to dosbox

• Now you only have to press the Play button in your Steam library to play the game, since Steam will run the Zandronum file thinking it is running dosbox
  
• Even after the dosbox change, your Steam profile will be still saying that you are playing The Ultimate DOOM counting how many hours you played and you can normally acces your Steam in the game
  
• After you run the game press Esc and go to setting and set your game by your taste, the resolution of the game should be not set to your monitor so you may have to set that on your own in the game with also some controlls that you prefere the other way

• Have fun

• There is a different guide I made for adding custom mods and maps for Ultimate DOOM

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=312875196

The only way I got the multiplayer going in this game is with bots yet. I can add a note for that if someone would want to know how. There may be a possible way to connect on servers inside the game by opening the console with ; button (the same one like in Source games) and trying to connect with the IP adress. But this one will work only with Servers that have the DOOM.wad on it and nothing else. Zandronum can run Doom online in many ways, sure, but mostly it runs on Doom 2 wad and it can't go to servers when you are in the game.