Many corner flashings, even if made from copper, are modified step flashings, cut in place to wrap the corner of the wall or chimney, while the leg running along the roof slope laps the apron.
The apron is bent to the roof slope. The angle is under-bent to provide tension, which helps the apron stay tight to the roof when installed.
The copper pieces are cut to accommodate two locking seams. Then, the corners of the apron piece are notched 3 1/2 inches in from the ends, the inside corner is cut at a 45-degree angle, and two 1/2-inch seams are folded over.
The folds on the apron and corner pieces are left slightly open to make it easier to join them together.
The seams are hammered flat and temporarily locked together with a small dimple created by a light tap on a nail.
The finished assembly provides a clean-looking transition from the apron to the sidewall step flashing and allows for some flexibility if the shingle coursing doesn’t line up with the dormer.