UPDATE: The Fallout London mod is out now, available to download from GOG. If you download the mod via GOG, it will work with Fallout 4: Game of the Year Edition from both GOG and Steam.

Mods for Fallout 4 from the developer Bethesda can be downloaded from its official website. To install a mod, you simply select 'Add to library' and load the game once it has finished downloading.