Press the center edge of each back piece under 1/4", towards the wrong side of the fabric and then 1/4" again to enclose the raw edges.
Stitch the hem down, keeping the stitches as close to the edge of the hem that is loose. This is a good place and time to experiment with needle positions and lining up you sewing machine foot, to obtain a straight stitch.
Press well to set the stitches.
