• Download and Install Fallout 4, from Steam or GOG
• Downgrade the game using their own Downgrader tool https://www.nexusmods.com/fallout4london/mods/1 Source and latest binaries here : https://github.com/Fallout-London/FOLON-FO4Downgrader A bit in panic at the moment, you may need to compile it yourself to fix some openssl version issues or to make it launch on Wayland. Consult the Issues tab on GitHub for common fixes.

(You can also downgrade manually, instructions at the bottom of the page. https://fallout4london.com/release/ There's also instructions on how to manually install the mod if you are having issues with the GOG Installer, there's ways to extract the raw files from GOG installer executables)

• Download and Install Fallout London. And select the correct game folder as asked.
• ???
• Profit ! (I'm installing it too at that moment on a Nobara(Fedora) machine. Will tell you if i had to do more than said.)