Recently came across this post on writing up a redacted document of all of the important info related to the world / story, and un-redacting things as the PCs discover. This lets them know what they don't know, and kind of the shape of what they don't know. https://ttrpg.network/post/20269477
Which reminded me of this well-known write up, Don't Prep Plots, which, while not entirely incompatible, is at least a very different approach.
Got me thinking of the way I do things, and a mix of all of the different things I have read. I try to run a pretty sandbox style game, but still have a lot of stuff going on in the world for the players to follow. In many cases the players will go towards something I haven't prepped or thought much about, and that improvised collaborative story telling lets me as the GM find out new information about the world right alongside the players.
I have started to think of this kind of gm prep as "Mad libs prep"
Mad Libs is a game where there are pre-written sentences, with blanks that need to be filled in by the players. E.g. "We get into our and to the beach" - players don't know what the sentence is when picking the words, so you can end up with that becoming "We get into our toaster and sleep to the beach". The idea is to have enough existing structure that things can get where they need to be, but with enough unknowns that can be filled in with whatever the players (who don't know the whole story) throw out there.
For GM prep, this can be knowing that there is an evil wizard who wants to take over the kingdom, and he needs to do it. The missing noun can be filled in by the players without them knowing.
For example, they become very interested in hunting for ancient magic artifacts? The essential is a legendary amulet and now the PCs are in a race against the mad mage to decipher its secret location.
Or maybe the PCs become monster hunters for hire, and the is the scale of a dragon or something similar, and the PCs run into the evil guys and uncover the plot.
Or perhaps the PCs really latch on to a side NPC that doesn't have much background fleshed out and becomes this person, who has some previously unknown connection to events that is discovered along the way (e.g. Martin Septim in TES IV).
The idea in general is to have enough material to know interesting things will happen, but not getting hung up on having every detail filled in. This also can be holding the things you do have prepared loosly, so maybe you had planned for the BBEG to have a secret lair in the mountains, but the PCs are really into a swampy forest area and end up wanting to spend all of their time there. Rather than "Ok, the BBEG has been up here uncontested the whole time and now the world ends, you all die" - the of evil layer is now deep in the wilderness, which can lead to a lot of changes, creating new lore, creatures, quests, etc.
Maybe all of this stuff is obvious but I am a relatively new GM and have mostly been figuring it out on my own. I'd love to hear other prep methods and tips!
I will do that if I can manage to get it done! π