We will not sew between these two safety pins. Make a backtack at the beginning and end of your seam. Now remove all pins. Reaching in between the pieced top and the backing to the opposite end inside the little quilt, pull it right side out through the space you left between those safety pins. Form the corners with a blunt object, not too sharp or you'll cut the corner.
Finger press the edges to flatten.
Turn the raw edges of the opening in even with the sewn edges and blind stitch in place. First we need to mark the plain blocks. With your long ruler and a fine line pencil, lightly mark all the plain blocks in a grid even with the 9 patch blocks. Using safety pins and starting in the middle, pin toward the edges about every 4 inches over the entire top, balancing any fullness outward toward the borders. Using the walking foot, or dual feed foot, start to sew in the center of the quilt in the seamline of a center block just to the border seam.