the simplest way would be to face the scallops, cutting 2 layers of the scallop border, sewing down the scalloped side and then clipping and turning carefully...large scallops would be easy to do this to... set the seams first (you always get a sharper seam line when the seam has been set), after, press the seam to one side as far toward the points as you can from inside.... now the scallops are sharp, finished on the edge and ready to be hand appliqued, close machine edge-stitched, or machine blind-stitched....it would also be possible to make this facing from iron on pellon facing, like a lot of applique is done these days. the important thing is to get those scallops finished on the edge befroe applying...good luck, this sounds lovely.... the corners might need to be played with a bit to make a smooth transition...