Prior to getting the dual rear valance I took the old valance off and took a piece of cardboard and used it as a cut out guide. Then measured the distance where the cutout side sits to have it sit the same on the non cut out side. Used a air cut out tool or I'm sure a sharp exacto knife and time will do the same. Once done took a light sand paper to remove the jagged edges and that was it! Only tip I would suggest is make sure to have a solid surface to secure the valance to when measuring and cutting out to avoid the valance moving.