It looks like you have it pinned well. You start quilting in the middle and work outward. I would stitch in the ditch with your walking foot straight down the middle vertically. I would then turn the quilt and stitch across the center horizontally. The big X through the quilt will help stabilize the sandwich. You can then alternate stitching outward making sure to change the direction of your quilting every other row. You alternate to prevent the sandwich from becoming skewed.