One way to put a professional finish to a garment is with Topstitching. Simple to do and only takes another line of stitching. The Sew with Mikki Kitset garments can be topstitched around the fabric casing for the elastic. Firstly by topstitching the seam down and secondly around the casing on the outer edge. By making the casing wider can then topstitch back from the edge and this will create a little frill. Have fun, make the casing really wide and topstitch back from the edge to ensure the elastic is firmly in place and it will become a ruffle.
