Cut a piece of fabric a couple inches longer and wider than the scalloped section. Pin the right side of the facing to the top of the quilt, letting the facing extend a half inch or so past the tips of the scallops. Working from the back of the quilt stitch a quarter inch from the edge of the scallops. Use a short stitch length, and pivot at the points. Start stitching about an inch before the first scallop, and end about an inch past the last one.
Now cut the facing to follow the raw edge of the scallops. The next step is grading the seam so it will turn and lay flat. Make sure that you don't cut the stitching. Using small scissors, try and cut the batting in the seam allowance out. Clip the tips off the pointy scallops and cut small Vs out around the rounded ones. The only thing that I can relate to is facing a collar in garment construction. This you tube video shows making collars with pointy ends and rounded ones and demonstrates both cutting off the pointy tips and cutting Vs in curved areas.
https://www.youtube.com/watch?v=dO1x76OceMo
At the base of the scallops, just make a single clip in the V down to the stitch line thru all layers, like in the binding video. You will have to do this on the outside scallops where they meet the straight part of the edge.
You can cut a big, long facing piece for the whole side of the quilt, but I personally would work piecemeal, cutting small sections for the scalloped areas, then joining straight pieces for the straight section.