I would cut that particular pattern piece including a generous seam allowance out of template plastic. Make sure to mark one side left (or right). Then you can overlay it on the fabric and draw each wing. Flip the template to draw the other wing. Cut out the wings and before you sew them, put the fabric in position on the right side. Flip it over along the matching seam. Put a tiny bit of fabric glue in the seam, then turn the block over and sew the line. This at least makes perfect sense in my head.