Before assembling the walls, I test fitted the garage doors. For the large windows above the two smaller garage doors I started with a Tichy storefront window casting and added some 1x3 strip to create a frame for the window. I also added a center post between the two upper windows.
Once I was satisfied with the fit of the garage doors I assembled the four exterior walls.
I had some 90 degree corner braces (I think they're from City Classics, but I'm not positive) so I used one in each corner, positioning them so they wouldn't interfere with the windows or roof.
Next step: the angled front door for the "Paint Store" section of the building.