When I world build in any game one of the first things I do is build the map. For Fallout I brainstormed some locations, checked to make sure I wasn’t stepping on canon too much (I’m okay with doing some), and then did some thinking about how I would change the location.
I decided go with Providence. I then looked at the map and thought about what fresh hell I can unleash on it, and I decided that a large portion of the bay front areas would be 50ft underwater. I haven’t decided what the mechanism is, but that’s not critical for me. I then printed a Google maps sheet and penciled in the new coast line.
Now that I have my rough map and defining geography, I started to think about what the impact of a sunken city would have on how people lived and behaved- from a Fallout POV*. Who would settle where, and how would they interact? What is the tone of the various sections, and how does it treat outsiders?
I take all these scribbled notes and then start to think about what kind of campaign it’s going to be. What is going to be the underlying thread that eventually connects the quests, and when will side quests contribute to that?
From that I loom at how that may change what I wrote previously about the location. How do I need to shift things?
After that the time consuming part starts. I’m not writing a whole source book that others will read, but I want to put my thoughts and ideas into a cohesive document to cement them in my head, and allow me to look back to get ideas.
Now it’s time for NPCs. I only do a few ahead of time. These are people important to a district. Mayors, gang leaders, military figures, traders, and maybe religious leaders. They help serve as a seed for story making. Other NPCs I prefer to form organically as the players interact with them. It just makes for more memorable “people” inhabiting the world.
The final part of world building for me is making final maps of the area and districts. Having a real city as a guide helps this a ton. Knowing distances helps me run a game, and seeing the maps helps players get into the setting.
After that I start making the first adventure. Having the world defined helps make that alot quicker.
Side note: One of the things that some players stumble on in test sessions was the fact that they are fixated on what the Commonwealth was like in the video game, and fail to realize that some shortcuts you have to make in that medium that do not have to be present in a TTRPG. The one I hit was how video games have to squish space. In Fallout 4 Concord has a total of maybe 15 buildings, and is a 5 minute walk from Lexington. When I pulled out some more close to reality numbers they were a bit confused why it took longer. The Fallout Commonwealth diverges from ours in many ways but setting thr scope was important to me.
Anyway, that’s my process. I enjoy it.
*Fallout, like most post apoc entertainment, takes a pretty grim view on humanity’s ability to reform functioning cities. I try to not let my own humble education on such things get in the way of fun games, so I keep things as “Fallout” as possible.