Step Two -- Glue the four wall pieces together. I usually attach an end piece to a side and the other end to the other side piece first (see picture above). Use a “jig” to ensure that your walls are flush with each other, perpendicular to the table surface, and dry firmly attached to each other. You can purchase small jigs through model railroad supply houses, but I improvise my own, using my trusty old tape measure. When these two L-shaped pieces are dried, glue the two halves of the exterior walls together (see photo below).
Before moving on to the roof itself, reinforce the interior of your model structure with a couple of carefully measured and placed pieces of heavy card or balsa wood. While not entirely necessary, these interior pieces will add strength, durability, and stability to your model buildings, preventing later warping. You can see an example of what I'm talking about in the picture just below this paragraph.
When the glue holding all of this together has dried, measure for and cut out the two halves of the roof. Again, take care to make sure your cuts are squared so that no annoying gaps appear at the seams – wherever one piece meets and/or butts up against another. Incidentally, the dimensions for the two roof pieces are -- length: 3 1/4" x width: 3". You can see my house with one half of the roof glued into place and the second half waiting patiently next to it in the photograph below.
The final part of this multi-part step is to glue the two roof pieces onto the tops of the four outter walls. Oh, an important caveat here that took me many years to figure out -- Always use less glue than you think you'll need. Too much is really messy. It will ooze down your walls and your roof pieces will slide out of place. You can always add a little more glue, but it’s difficult to clean an excess of the stuff from your cardboard walls successfully.
At this point, you now have your basic structure. Job well done! Take a lengthy coffee or tea break, work on something else for an hour or two, or stop for the evening before returning to add additional external details later. I usually like to have a couple of different small projects happening simultaneously. That way while one thing is drying, I can paint or trim mold lines from figures, like the Spencer Smith cavalry squadron sitting on the cutting mat next to the merchant's townhouse under discussion here. Anyway, we’ll discuss adding chimneys tomorrow. Happy modeling!