Use a blind stitch, not a slip stitch, to sew the binding to the back. As Patrice said, it's the same stitch used for applique. With thread that matches the binding, pick up just one or two threads of the backing fabric exactly underneath the edge of the binding, then slide your needle into the fold in the binding. Bring the needle back out of the fold about 1/8 inch away from where you entered. Repeat. The closer together you make the stitches, the more durable the binding will be.