reversia.ch says:
For those that create custom settings - how much do you prepare / write in advance? Broad stroke and then fill the blanks as the game starts or detailed write-ups for majority of the locations, groups, etc just in case players end up interacting with those (and to world-build, I suppose)?
Sometimes it's just one idea- one city, one "thing" and then you build a whole game off that-- I did one that was just "a game in an Arthurian style based in a castle" and it built out slowly from there. Another time it was a specific city with 9 guilds, and I did a quick and dirty map showing some locations in the city on a yellow pad.
But my main setting I've been working on, writing in, and running in for over twenty years- played AD&D 2nd edition, DND 3.5, Pathfinder, and 5th Edition in it over the years, and played in many locations for a lot of games, plus the novels I am working on for it, so it has a ton of detail which is hard to generate quickly.