1. Measure how wide your piece of fabric should be to cover your bust. Cut away this amount of fabric along the bottom of your skirt
2. Open up the stitching at one of the side seams using scissors or a quick unpick
3. Fold your piece of fabric in half and line up the halfway point with the back of the skirt. Hand stitch the two pieces together along the back. Finish the bottom of the skirt part by sewing a new hem.