This guide covers mod installation on Steam version, on Windows. It can be useful for Bethesda version too.
Instructions remain the same for Linux, but you'll need to download different mod injector.
UWP version that can be accessed via Microsoft Store or Xbox Gamepass doesn't support modding. Modding tools for UWP version do exist now.

The new updates for Doom Eternal (aka Ancient Gods Update, aka DE v3.0+) breaks Modloader and mods
Make sure you download newest versions of these.

You only need EternalModInjector that combines all the tools necessary.
Windows & Linux[discord.com]
UWP[discord.com]
If links are not available for you, go to DOOM Eternal Modding Discord (link below) and check #eternal-tools channel.
Discord Server[discord.gg]

Horde Mode v1.42 by Proteh
Replaces main campaign with long and ballbusting set of enemy encounters.
Nexus Mods[www.nexusmods.com]
Discord Direct Link[discord.com]

Horde Mode - Trial of The Dark Lord v1.41 by Proteh
Worthy sequel to Horde Mode, with new enemies and new mechanics like Meathook puzzles and pseudo-Invasion.
Nexus Mods[www.nexusmods.com]
Discod Direct Link[discord.com]

Enemy Randomizer by Proteh
This mod randomizes every enemy encounter in the base game and the DLC, including secret encounters and slayer gates.
Consists of NG version (demons spawn with regular progression in mind), NG+ version (you get all weapon from start, and hub area is cut) and unbalanced version.
Nexus Mods[www.nexusmods.com]
Discord Direct Link[discord.com]

Original TAG1 by PowerBall253
Un-does the nerfs to TAG1 (listed here[discord.com]) delivered with DE v5.0 update.
Discord Direct Link[discord.com]

F off Escalation Warning by Proteh
Does exactly what it says - removes those annoying pop-ups when you approach Gore Nest for second wave of Escalation Encounter
Discord Direct Link[discord.com]

F off Campaign Tutorial by Proteh
Removes that annoying pop-up when you select the difficulty.
Discord Direct Link[discord.com]


Other mods by proteh[www.nexusmods.com]

Doom Eternal Modding Discord[discord.gg]

You can easily access main folder of Doom Eternal by going to your library -> right clicking Doom Eternal -> Manage -> Browse Local Files.

1. Download 'HordeMode_v1.42.zip' and all the tools
   
2. Extract 'EternalModInjector.zip' into
   
   ...\Steam\steamapps\common\DOOMEternal\
3. Place 'HordeMode_v1.42.zip' into
   
   ...\Steam\steamapps\common\DOOMEternal\Mods\
   Extracting mods isn't necessary.
   
4. You should end up with this:
   
   -/DOOMEternal/EternalModInjector.bat -/DOOMEternal/base/DEternal_loadMods.exe -/DOOMEternal/base/DEternal_patchManifest.exe -/DOOMEternal/base/idRehash.exe -/DOOMEternal/base/EternalPatcher.def -/DOOMEternal/base/EternalPatcher.exe -/DOOMEternal/base/EternalPatcher.exe.config -/DOOMEternal/Mods/HordeMode_v1.42.zip
5. Run 'EternalModInjector.bat'. It should make backups of original assets, replace them with mods and launch Doom Eternal.

This instruction works with all mods for Doom Eternal.

1. Remove Horde mode save slot or back it up, or else it will become corrupted and cause problems with the game.
   
2. Remove contents of 'Mods' folder
   
3. Run 'EternalModInjector.bat'

Allstin's guide on save slots manipulation

21.03.25 - Updated links for Randomizer and Original TAG1 mod
21.03.22 - Updated links for Randomizer
21.03.21 - Updated links, added other worthy mods
20.10.26 - Total overhaul, links for new versions of modloader and Horde Mode added
20.09.13 - Guide written