Insert the crochet hook under the two top loops of any edge stitch. Yarn over, then pull the hook through the edge stitch. Yarn over, then pull the hook through the loop on the hook. Pull the tail end of the yarn to form a knot, attaching the edging yarn to the crocheted piece. Do one single-crochet stitch in the same stitch.
Make the first scallop by placing one double-crochet stitch in the third space from the hook. Chain one. Crochet two more double crochet stitches in the same stitch, with one chain stitch between each double-crochet stitch. Chain one. Skip two stitches. Do one single crochet stitch in the next stitch.
Continue crocheting the scalloped border until you are 3 or 4 inches from the corner, Because this stitch pattern is flexible, you can contract or expand it slightly to fit the space you are working in. The last single crochet stitch should be in the second space before the corner hole. To crochet the corner shell, chain one. Do five double-crochet stitches in the corner hole, with one chain between each stitch. Chain one. Single crochet in the third stitch from the corner on the second edge of the border.
Continue crocheting around the border. When you near the starting point, adjust your stitches so the last scallop will meet the edge of the first scallop. Slip stitch the last scallop to the single crochet stitch at the starting point. Fasten off the yarn and weave in loose ends.