The recipe, such as I remember of it:
I started by flipping through the textures until I came across the hachure one of the gray lines, and filled the page with that using the paint can once I had set it to pour the pattern using a check box in the tool options palette on the right.
Using the pencil_texture brush tool, I sketched in the room and hallway background shades. I sketched in the outline lines simply using a variety of of the brushes set to a few pixels wide, liberally using control-Z undo until I got them outlined simply.
But that looked really boring and had slightly wiggly borders since it is harder to draw nearly straight lines with either mouse or tablet than a pen on paper. So, to add interesting doodads that inspire DM creativity, provide shadows for thieves to hide in, etc, and primarily break up the crappiness of the initial outlines, with a couple of small pen tools I went back around the outlines and reinforced one here, made little bits and interior walls stick out there, put some rectangles that could be boxes or chests or furniture in other places. Added some details to the floors as part of this. Whenever I had to change back to the not-quite black for outlines after doing something else, I used the eyedropper on one of the fatter lines to pick it back up in the same color.
Went back around with an airbrush tool that I thought I had white in but was really a very pale green offwhite (which I think intensified a bit in saving as a JPEG) to trim in the hachure background to be just around the dungeon, except for a few bits I missed at the edges. A few touches of red spatter spray for blood of prior violence and green stippling with a small brush for green slimes or something like that pretty much wrapped it up.