Review and revise
When you think you're done, read through your write-up. Is anything missing? Add it. Is anything no longer needed? Cut it.
Make sure it makes sense. What was the site originally built or used for? Would it have served that purpose? How is it being used now, and does that actually work? How do denizens come and go? How do they see? Where do they sleep, store food and prep it? What do they do with waste?
Do the site's areas, layout, and contents reflect the answers to these questions? If not, or if you can't answer these questions at all, then revise your write-up accordingly.
Give the write-up one last read-through, looking for inconsistencies or conflicts. If you find them, fix them.
It'll never be perfect. There's always something you'll miss, something that could be better. It's okay. If anyone even notices, you can fix it during play!
I'm ready to write up the Green Lord's tomb! I start by turning my nodal diagram into an actual map of the main level, because it helps me to picture the place.
From there, I start detailing the "Exterior," but quickly realize that I need to break it up into the approach, the exterior of the site proper, and the various entrances. I give the crinwin their own write-up, because I want to detail their numbers and tactics.
I move on to the interior, starting with the entry chamber. I realize that I'm writing things that are true about every room, so I create an entry for "Interior (throughout)."
I write up the collapsed tunnel and its secret burrow, then the central chamber. The central chamber starts simple enough (vaulted ceiling, smashed sarcophagi, empty niches), but I realize that this is where the rebel Fae would have burned the bodies of the Green Lord's servants. I add details about the soot-stained ceiling and scorched floor.
I write up the shrine next. I envision its original state, then I imagine what Sajra has done to it. He's replaced the goddess's statue with his hoard, so I add "broken statue" to the rubble in the central chamber. I write up Tiwlip and Wynfor (the kids) and some notes about how they live. Then I write up the spirit of the spring (because Blodwen can speak with spirits).
Next comes the mossy chamber. I create the moss spirits as a bit of comic relief. Then I realize that moss needs light and water, so I add the glowing makerglass in the ceiling and an irrigation channel in the floor. (I add the channel to the map, the shrine, and the central chamber, too.)
I write up the great hall, starting with the pillars, the crinwin nests, and the stairs. I want to include an "unstable construction" hazard, telegraphing it with the collapsed pillar, the rubble, and the sagging ceiling. Then I prep the hazard as an impending doom and grim portents.
I'm yet to decide where Pryder is. I know he's asleep in a burial niche, but where? I pick randomly, placing him between the mossy chamber and the great hall. I write up that hallway, with the crinwin who kidnapped Pryder jealously guarding him and some "trusted" crinwin spying on them.
Speaking of crinwin: They get a second entry for the interior. I consider making a "crinwin encounter" table to reflect the weird stuff the crinwin get up to, but the crinwin are almost certainly going to be on alert when the PCs enter the tomb. Instead of a table, I write up some if/then tactics for them and a quick list of things they might be up to if caught unawares. I also add some details to other areas to reflect the crinwin's relationship to wasps, their obsessive hoarding, and their attempts to ape Sajra.
Sajra also gets its own write-up. I realize that it might not be in the tomb when the PCs arrive, so I create a Die of Fate table for that. I start writing some if/then tactics, but Sajra is smart, mobile, stealthy*,* cautious and magical*—there are too many options! I opt for a brief description of its priorities and likely tactics.*
I don't bother with a map or detailed writeup of the inner tomb just yet. I picture it as a fairly linear space largely separated from the main floor. I doubt the PCs will head in there this session. I'll improvise if they do.