I first cut the arches for the shutter and attached them to the dome ring.  I used temporary spacers between the arches to maintain the distance and shape for the shutter opening.  I used 5/8-inch exterior grade plywood for the arches and ribs and doubled them for a thickness of 1 1/4 inches.  After the arches were in place I attached the ribs.  I began by attaching the two center ribs in order to support the arches.  I then attached the remaining ribs alternating from one side of the dome to the other.  All the arches and ribs were attached to the dome ring with glue and screws.