Take some heavy duty glue
Make a pretty pattern (remembering to get close to corners and edges, as sometimes you need practicality over pretty, especially as this bit won't be seen).
Stick the roof box to the flaps of the walls box (hopefully you can see what I mean from the photos - there's not many words in the English language that distinguish one bit of box from another!)
Add some windows. Mine are 20cm square, with a 2cm wide cross section-y bit. I also left some cardboard where I will put my window box and flowers (again Swiss inspired).