Cut out two of the bottom wall; in opposite corrugation directions and glue them together. Then cut out the doorway.

Test it for fit, trim as necessary, and glue it into the bottom section. It is nine inches from the stairwell.

Now let's install the upper wall. This is the bathroom wall. This wall is a little bit more complex of a fit because of the slant of the roof. I recommend you cut a single piece out of cardboard and trial fit it in place. That way you can adjust/modify and re-cut it as needed to get a nice fit.

Once you have the shape just right use that piece to trace a second cardboard piece, with the corrugations going in the other direction. Then glue those two pieces together, cut out the doorway and glue it into the dollhouse.

Glue it 15 1/2 inches from the left wall.

Next we finish off this section of the dollhouse by gluing in the floor of the attic. This piece is pretty flexible. It should go approximately 8 inches above the second floor. Trim it as needed for a clean fit.

This next picture shows the completed back section of the dollhouse.

Let's continue and work on the front section of the dollhouse