Check out the pictures of this project.
The arches are pretty simple. The main concept is that the piece that forms the underside of the arch starts the process. From 1/4 inch luan plywood, I cut a piece long enough so that it could be sprung into place between the two posts. I actually added a little temporary ledger board on each post so the sprung board was supported. I added drywall nailers along the ceiling and on the posts above the arch.
I then installed nailing blocks along the arch board about every 8 inches to a foot. Next install the drywall on one side of the arch and screw it into all the nailers, repeat on the opposite side. Next drywall the bottom of the arch by gluing on a spring piece of drywall to the bottom of the arch. Keep screws in the bottom of the arch to a minimum, and let the drywall form a fair curve and get help in place with the glue (construction adhesive). They make a special corner bead for arches - get some of that and plaster the bottom of the arch and the sides.