When I did mine I taped together telephone pages the length of each side. I use a desert plate for the corners and folded the taped pages a side at a time to get the spacing even. I used the creases to center the plate and then reversed the plate for the inner part of the wave between.
When I had my edges just right, I cut out the outline. I pinned my paper to the quilt straight edge and drew along the paper edge with my marking pencil. I stitched the binding to the drawn edge before cutting the wave edge. There is a rotary cutting ruler that allows you to cut the wave edge but since I was just doing one quilt I didn't buy it.