Here are the finished valances:
She bought white cotton pique for the main fabric and royal blue broadcloth for the contrast. Her valances were to be 19" deep. In order to avoid having seams in the applied design, I worked out the design using spare ribbon and determined that it would take a continuous 4.5 yard length of the contrast fabric. I cut 3" lengthwise strips and folded each edge in to the center for a finished 1.5" strip.
Then it was just a matter of careful measuring and pinning the miters in place. My gridded cardboard cutting board came in very handy for this since the grid lines were visible through the pique and made it easier to line up the horizontal and verticals perfectly. I pressed the pinned fabric strips in place, then topstitched. They are interlined and also have a layer of fairly stiff interfacing behind the flannel to make them extra crisp.
Here's an "in process" shot with a close up of the miters.
This was a labor intensive but very fun little challenge.