From the How-to on Nexus:
https://www.nexusmods.com/fallout4/articles/3115

BUFFOUT 4 INSTALLATION - PART 1 | YOU NEED TO DOWNLOAD AND INSTALL THE FOLLOWING

I. Fallout 4 Script Extender | Modder's resource that expands the scripting capabilities of Fallout 4. This is also a hard requirement for the vast amount of other mods.
Installation: Download the latest build (0.6.23 or newer) and extract all contents inside f4se_0_06_XX folder into Fallout 4's Installation Folder, where Fallout4.exe is located.
--> Can be skipped, the Fallout 4 Script Extender is already included with Fallout: London

II. Microsoft Visual C++ Redistributables AIO | All-In-One package that contains program libraries required for any and all PC games.
Installation: Download the latest release and run VisualCppRedist_AIO_x86_x64.exe, restart your PC after installation.
--> Can be skipped, it's VERY likely you already have this installed, but it can't hurt to re-install anyway

III. DirectX End-User Runtimes June 2010 | Old DirectX libraries that can help prevent audio related crashes.
Installation: Download the main package and run it, then select any location to have it extracted. After it's extracted, run DXSETUP.exe.
--> Can be skipped, for the same reason as above, but it can't hurt to re-install anyway

IV. Address Library for F4SE Plugins | Provides version independent resources required to run several important mods, including Buffout 4.
Installation: Manually download the main file and place its version-1-10-163-0.bin file into the Fallout 4\Data\F4SE\Plugins folder (create it if there isn't one).
--> Can be skipped, the Address Library for F4SE Plugins is already included with Fallout: London

BUFFOUT 4 INSTALLATION - PART 2 | INSTALL MANUALLY (WITHOUT A MOD MANAGER)

1. Download the Buffout 4 Main File and extract the archive somewhere on your PC.

2. Place / move the extracted F4SE folder into your Fallout 4/Data folder (& overwrite if asked).


Start the game via the f4se_loader.exe (not Fallout4.exe or Fallout4Launcher.exe*) located in the main Fallout 4 directory

*Only run Fallout4Launcher.exe for configuration purposes ("Options"), do not start the game by clicking "Play"

Thats exactly what i did but it made me have an f4se error and wouldnt start. saying f4se needed elevation i dont remember the exact wording because im in the process of a full reinstall because i couldnt remove the error by any means. Thanks though

Better to just start over from scratch then, as in doing a clean reinstall of both Fallout 4 and Fallout: London.

I did this (easy mode):


IT WORKED!!!

I follow this:

BUFFOUT 4 INSTALLATION - INSTALL THESE FILES MANUALLY

1. Download the Buffout 4 Main File on NEXUS

and extract the archive somewhere on your PC.

2. Place / move the extracted F4SE folder into your Fallout 4/Data or Fallout 4 Data folder (& overwrite if asked).

And:

inside the buffout 4 zip is a folder called F4SE. If you go into your fallout 4 install folder -> data folder, you’ll see another folder called F4SE. Overwrite that with the one you just downloaded. It will insert the files you downloaded to the fallout folder without deleting existing ones.

Finally:

Go into the fallout 4 folder. There is a f4se_loader (fallout 4 script extender) .exe. I created a shortcut to it on the desktop, went to compatibility mode and chose to run as administrator. Renamed it to Fallout 4 London because I like to know what I’m looking at. Launch the game through this instead.

JUST REMEMBER! If you've got a Nvidia videocard, turn of weapon debris (fallout e ini or fallout 4 launcher option).
It's been crashing the game for years and Bethesda/Nvidia haven't acknowledged it at all.

Ok i will try this tonight and yes i have a 4090 so ill turn of weapon debris too ty

had to redo this 3x before i got a install that launched, including nuking the appdata foilder