Cut the existing hem to about hip height. I made the front of my shirt slighty shorter than the back - giving it a dipped hem feel.
Cut out a band of rib wide enough to fit sunggly on your hips. Sew both ends of the band together and then fold in half so that the raw edges are together. Pin the rib band to the shirt hem, stretching to make it fit. Zig zag stitch them together.