Installation

Video tutorial here (@guesty5060 you're welcome) (might already be outdated, see text guide)

Make a copy of ULTRAKILL so you can safely mod that version without affecting your normal game:
1.1 Locate ULTRAKILL in your Steam library (or wherever you store it)
1.2 RMB -> Manage -> Browse local files
1.3 Make a copy of the ULTRAKILL folder and put it somewhere
Get & Install latest stable BepInEx 5 that matches your CPU architecture into the new ULTRAKILL folder using their guide. Make sure to run BepInEx'ed ULTRAKILL at least once so it can generate needed folders & files.
Download one of releases, unarchive and merge both ULTRAKILL folders.
Run it once, wait for the error message "unable to find config file, generating a new one" to appear in the console then exit.
Go to BepInEx/plugins/VRTRAKILL, open VRTRAKILL_Config.json and set your preferences there.
Run VR-ed ULTRAKILL again.
For your own convenience you can Add ULTRAKILL to SteamVR library
Add the modded ULTRAKILL as a 'non-steam game': 1.1 ADD A GAME (at the bottom left in the library) -> Add a non-steam game -> select your VR-ed ULTRAKILL.exe
Right click on the new VR-ed ULTRAKILL in your Steam library -> Properties -> Shortcut -> Include in VR Library
Open SteamVR. Now ULTRAKILL should show up in there.

Building from source

Video tutorial here

Download VRTRAKILL source code (as zip file from master branch or from releases)
Open the solution file (.sln)
In the Plugin project:
- Enable unsafe code
- Change the Debug thingie on top to Release
Build the solution (RMB on VRTRAKILL -> Build)
Everything should now be in the solution_folder/bin