There are four small windows in the back of the building, and one on the side. The other side can't be seen in the photos, but I decided to put one there as well. Again, the location of the windows is approximate, matching what I see in the prototype photos.
After cutting out the windows, I wanted to simulate the siding used on the building. To get nice straight lines, and to make sure that they matched on the sides of the building, I put all three pieces down on my glass work surface and used double-sided tape to hold them in place. I then used the dental pick you see in the photo to scribe the lines. The lines were marked at 9 scale inches apart.
I noticed that the back wall was not flat anymore, so I glued some more square styrene strips to force it to stay straight. I also attached the triangular pieces to make sure the side walls stayed in place.