I got a little more done on this project last night, and got the walls trimmed and assembled.
To do the corner trim, I had actually cut the sides just a little short to account for the 4″ corner trim, and as the siding is .040″ thick sheet, I cemented .040″ square strip to the ends of the sides and represented the corner trim that way. The sides and ends are thus assembled in the normal way, with the corner trim becoming part of the side. You can see in the photo above that I used pieces of 6×6 to brace the inside of the corners.
To trim the door opening was done in much the same way, carefully filing everything square and cementing .040″ square strip around the inside of the opening. To ensure that the opening is the right size I measured and cut the blanks for the doors (which will be the next part of the project) and used that to make sure that I filed the overall opening to the right size. The door plus the trim framing should all fit snugly into the opening.
All that remains now of the basic shell of the shed is to file/sand the tops of the sides down to the gable profile for the roof, and to of course build the roof. Then add the doors and windows.