After wallpapering my first Sylvanian Families house, I decided to add wooden trim to frame the room and give the walls a little dimension. I used tiny strips of balsa wood, which is very soft and can be cut with an X-Acto knife.
Since I am not that precise, I held the strips up and roughly measured them against the house. If you are more precise, I would recommend taking exact measurements of the windows and walls, and translating that onto your balsa strips.
After I roughly measure each little piece, I follow the angles on my cutting mat to have mitered-looking edges on the windows.
When put together, it looks like a little picture frame. At this point, I would line it up with the 90 degree angles on my cutting mat to make sure it was square. I did this for each window, and then dabbed a bit of rubber cement in the mitered-looking corners to hold it all together.
Once I had all pieces cut, I applied a thin, brown wash with paint. This let some of the wood grain show through.
These are all the pieces I used for my Cozy Cottage: on the far left is the door frame; next are all the loose sticks for the wall trim; then finally, the windows. For the upstairs windows with curves, I used a bigger sheet of balsa and cut out the round parts as one piece.
With the pieces dry, I painted the backs of them with rubber cement and gentle pushed them into place with tweezers.
The process went surprisingly fast. I did my first house in just an evening, and the difference is huge! It looks much more like a real, finished home.