So let's start with an easy base. I begin with a background layer that's a textured neutral - you can grab these from parchment or rock textures over at cgtextures.com. I then just quickly drawn in a wall outline that will be the room. The wall lines are done on a new layer - I always keep my lines on a separate layer on their own:
With the walls in, I use the magic wand tool to select the area that will be the walls (and use Select->Modify->Expand to make sure the selection runs down the middle of the ink lines). Then go to the Paths dialog and click the 'Make work path from selection' button to make a path from the selection. (also use Select->Save Selection before going any further - you'll need this selection later)
This creates a work path. Drag and drop the work path to the New Path button to create a permanent path. You also want to invert the selection and do the same process to get a path that surrounds the floor space. You'll now have two paths.
This is the structure that will underpin the work that comes next.