I spent 3 days trying to get FOLON to work following both the official website guide and every post I could find on the internet, and I finally said fuck it and played around with it myself and figured out how to get it running.

I'm posting here in case anyone else has had the same problems I did and don't have a solution. The first half is pretty much identical to their instructions

This does not use the FOLON Downgrader, Because I dont like the idea of signing into my steam account through someone elses program.

Also don't be in a steam beta. Use release steam

• Run steam in console mode as per instructions here. On Linux just close steam and run "steam -console" in terminal
• Download all the depot packs like they instruct.
• Create a folder elsewhere and go to where the files downloaded and empty each folder into the created folder so you have a full build of the last version of Fallout 4 (Do not alter this directory further, unless you want to have to go through all that pita downloading again, so only copy files from here, do not move them from here)
• Go find appmanifest_377160.acf in /steam/steamapps and make it read only
• Go to your /steam/steamapps/common/Fallout4 folder and delete everything it, and copy the Fallout 4 you just downloaded into the folder.
• Launch fallout once, i dont know how far you have to go, i typically go to character creation, to make sure the game creates all the necessary files
• Go to GOG and download the Fallout London files. should be 10 files, one being an .exe and the rest being bin files
• This is where we diverge. Run protontricks, If you don't have it installed, then read install instructions here and select Fallout 4
• Ignore any missing file arch 65 errors, keep clicking okay until the "What do you want to do" window pops up
• Click install application, Ok
• In this next window, click cancel
• In this window, select Run an Arbitrary Executable (.exe/.msi/.msu) and click okay.
• In the file explorer that pops up, navigate to your FOLON installer (at the time of this posts edit it is ~~setup_fallout_london_2.31_revision2_(64bit)(75054)~~ setup_fallout_london_v1.01_iv2.31r3(64bit)_(75422).exe) and run it
• Select your desktop as the destination (Don't select your Fallout 4 folder, It'll just install FOLON into a Fallen London subfolder)
• one the install is done and over, select every file in that folder and copy (don't move, copy, You want to save this folder as well in case something screws up so you don't have to through the install steps again) the Data and src folders (this is just f4se source code folder, but it'd go in /Fallout if you installed f4se manually anyway) and all loose files beneath it to your fallout 4 folder, and tell your file manager to merge folders and overwrite files.
• Move the files in _Appdata to ~Steam/steamapps/compatdata/377160/pfx/drive_c/users/steamuser/AppData/Local/Fallout4/, overwriting what is there
• Move/Fallout4/ files in _Config to ~/Steam/steamapps/compatdata/377160/pfx/drive_c/users/steamuser/My Documents/My Games, overwriting whats there.

You should be able to launch Fallout 4 now, and have it load directly into Fallout london, and thanks to making appmanifest read only, steam will never update the game and bork your mod.

Now there are still lots of bugs in FOLON, like the train crash outside the starter area, which most people fix by installing the Buffout mod, but thats beyond the scope of this post

If any of you out there had woes getting FOLON to work like I did I really hope this helps

Edit- About 30 minutes ago, as of this edit, a patch was released to address many problems with FOLON. None of the techniques I've listed here for installing the main mod works with the patch. I am continuing to experiment and if I can figure it out I will edit this post again or make a replied post.

Edit 2-

Okay, I could not find a way to get the patcher to work to patch the game

But the base install files are also updated to the same version as the patch, so just redownload the executable and the bin files from gog and follow the steps in the original guide