Establish areas/rooms

Write out the distinct rooms or places that are suggested by your site's foundation and story, by its denizens, and by common sense. A small site might have just one area, but most sites have at least a few.

If the site was built for a specific purpose, start there. What rooms or areas would it have had to fulfill that purpose? A home would have a place to sleep, a hearth, some sort of common room, a larder, etc. But a fortress would have ramparts, a gate, a well, maybe an inner keep.

Think about why the PCs are here in the first place. If they're looking for something, what room or area is it in? Why is it there, specifically? What does that tell you about the site?

If you need to invent a new area or room (to hold a danger or discovery, or to just fill out the site), then create it as a new site using the tools in Book II. If the PCs are exploring a Stone Lord ruin and you need to place a broken construct somewhere, then roll up a new Stone Lord site, treat it as an area within the larger site, and place the construct there.