"draw the rest of the worm"
i read His Majesty the Worm! game's got plenty of buzz, you don't need my rec, but I come out of the book appreciating how much it explains how to use OSR trademarks like dungeon maps and random encounter tables. "integrate your random encounters into whatever spot on the map they show up, and swap them out with new events when the party returns to the surface" is really really helpful procedural advice, albeit one that necessitates a lot of preparation. And to its credit, it breaks down exactly what you need for that preparation really well. Once you have your city and underworld maps and their respective event tables, it gives you a ton of material to work with and dominoes for your players to knock down.
However
The dungeon creation advice itself left me a lot colder. It has a process for determining the macrostructure, but each individual floor kinda reduces to "draw maps, come up with a bunch of interesting things to happen." Even the "dungeon seeds" in the back have maps, but don't have room by room breakdowns. Which is the point, they're a starting point for inspiration, but the end result is that you still need to draw the rest of the owl when preparing things yourself.
His Majesty the Worm does a better job of showing me what a megadungeon needs than any other game I've read. It's bridging the gap between where you start and what it needs that's tricky for me.