Lace up the cord you made through the loops. Try on what you have so far at this point to determine how much elastic you will need for the back. Measure the distance between each side of the bodice on your back, and cut two pieces of elastic that same length.
Stitch the ends of the elastic down on each side. Fold the top of the skirt over, and sew the casing shut.
You should leave about 1/4 inch seam allowance of the fabric on the elastic casing. Once you are done sewing the elastic and casing, sew an overlock stitch all the way around on the raw edge to prevent fraying.