I would do a very gentle wavy pattern, repeated every 2" or so from the left edge to the right edge, running from top to bottom.
I have attached a very rough sketch. Your lines would be smoother and in white and maybe closer together. You could do this with a walking foot. or free motion. Just be sure to mark your straight lines well so you stay straight .
Watson
Last edited by Watson; 08-28-2020 at 04:34 AM.