Bethesda's port is bad. V-Sync affects mouse input, game logic is not functioning properly, mods from the mod browser are not actually fully compatible and bethesda wants you to have a bethesdanet account.

This guide will show how to (reversibly) replace the DOS launch option of the game with something that is much more stable and hassle-free than the Bethesda port.

There are two options at launch:
Play DOOM + DOOM II - Bethesda's new port;
Launch DOS version - Launches DOS version with simple frontend.

We will be modifying the latter, so you can still access the new port if you want.

Pros::

• A small, fast "launcher" through which you will be able to run any WADs you want.
  
• An "it just works" way to play a game - everything is set up already, the only choice you need to make is whether you want widescreen image or not :P
  
• Mild graphical improvements that work out of the box - moving liquids, corpse mirroring, proper blood color for different enemies, etc.
  
• No Bethesda.

Cons:

• No achievements. Launch the new port to earn them!
  
• No mod browser. You'll have to get your mods from websites that host them.
  
• No multiplayer. Although you can additionally install a sourceport that will support it.
  
• No easily accessible extra difficulty options, such a pistol start and fast monsters.

First we need a "launcher" that will allow you to comfortably choose which WADs do you want to run. No command lines, no .bat files, no Bethesda.

1. Get the latest version here.[github.com]
   
2. Open the .zip file you've just downloaded.
   
3. Open Explorer, locate your Steam folder open the following path Steam\steamapps\common\Ultimate Doom\dosbox
   
4. Paste ZDL.exe into the dosbox folder.
   
5. Inside dosbox folder, launch ZDL.exe. Make sure that it launches properly. Leave a comment here if it doesn't, and make sure to specify what kind of error you've got.

Doom Retro is a modern sourceport that will let you play the game with little to no issues. Refer to the first section to see what you'll gain from it.

1. Visit the official website[www.doomretro.com] and press the Download DOOM Retro button on the right.
   
2. In Steam\steamapps\common\Ultimate Doom\dosbox directory, create a new folder and name it doomretro.
   
3. Paste the contents of the downloaded .zip archive into the Steam\steamapps\common\Ultimate Doom\dosbox\doomretro

Now we need to create folders that will help us to sort game data and mods.

1. In Steam\steamapps\common\Ultimate Doom\dosbox directory, create two folders. One must be named IWAD and the other one's name is PWAD.
   
2. IWAD will contain wad. files that contain all the data you need to run the game. You may think of them as "master files". In the Steam\steamapps\common\Ultimate Doom\rerelease folder, there are two - doom.wad and doom2.wad . Copy them and then paste inside the IWAD folder.
   
3. PWAD will contain .wad files that modify require an IWAD file to run. Basically, they are mods. From the Steam\steamapps\common\Ultimate Doom\rerelease directory, copy and paste id1.wad (that's the new "Legacy of Rust" campaign!), masterlevels.wad, nerve.wad (that's the "No Rest for the Living" campaign), plutonia.wad, tnt.wad and sigil.wad to the PWAD folder.

Tip: if a mod that you'll download will have more than one file (probably a .deh one), make a separate folder for this mod to avoid confusion in future.

We're almost done, Time to set our ZDL instance up.

1. In the dosbox folder, launch ZDL.exe
   
2. Open the General settings tab.
   
3. Under the Source ports"field, press a + button. An Explorer window will open. Find the doomretro.exe file in the doomretro fodler and double click it. Now you will see that DOOM Retro is displayed in the Source ports field.
   
4. Under the IWADs field, press a + button. An Explorer window will open. Find the doom.wad and doom2.wad in the IWAD folder. Select both and press the Open button. Now both your IWAD files are displayed in the IWADs field.
   
5. Open the Launch config tab.
   
6. In the Source port field, choose Doom Retro.
   
7. Let's launch the Legacy of Rust campaign. In the IWAD field, select Doom II. Legacy of Rust requires Doom II to run.
   
8. Under the External files field, press the + button and select id1.wad in the opened window. Double click it.
   [o/list]
   
   This is how your ZDL window should look like now:
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   If the parameters on the screenshot don't match with yours, you must have skipped some of the steps.
   
   If it looks as good as my screenshot, go on and press the Launch button. Play around with the screen size in the options and go have some fun!

Don't worry, this is easily reversible. Only do this if you want to launch your new setup from Steam, so you could get these sweet 2500 hours logged while playing.

In the dosbox folder there is a launcher.exe file. Rename it into a old_launcher.exe. This file is what launches the DOS launch option. Now rename ZDL.exe into launcher.exe. There you go, you can launch this setup from Steam now. If you'll want to play the DOS version, undo these renames. Everything in this guide is completely reversible and you're not ruining your installation :D

By now you probably understand how to use the launcher - choose the IWAD and then add the PWAD files you'd like to run. Some PWADS can be mixed together. Most don't. Feel free to experiment with that.

You might want to run something like Brutal Doom mod. ZDL can do that too, however you will need the GZDoom sourceport for that, which can be installed as easily as the Doom Retro.

If you're wondering why I haven't just told you to install GZDoom right away - GZDoom is not exactly "vanilla". Vanilla-compatible WADs are better experienced with something like Doom Retro. Do experiment though and try running GZDoom with vanilla-ish settings and Smooth Doom mod - it's fun too.

For more IWADS and PWADS I recommend visiting https://www.doomworld.com/. Believe it or not, but you don't need to register there to download the mods :D