You can purchase wavy rulers. My dear sister got me one for Christmas this past year, but you can do your own wavy lines. Cut the width of your fabric wider than necessary. Lay both pieces right side up. Then cut thru both layers with your rotary cutter in a wavy line. Don't go too far in your dips, or it's very hard to piece back together. Then take the top right side and the left bottom piece - put right sides together and start sewing together - very slowly. Once you have them pieced, clip the curves, iron out and use your ruler to square up (hence the not to cut the pieces wider than you want the finished piece to be). Then sew the left top side to the right bottom side.
I think is what you were asking for.