1. Undo the old seam of the hem carefully by using a small pair of scissors, a seam ripper, or cutter.
2. Put on the pants. Fold each pant leg inward to the length that you desire. Keep in place using pins or masking tape.
3. Take off the pants. Turn them inside out. Measure the length of excess fabric you will cut. For formal pants/slacks, make sure the hem has a fold of 1 inch and for jeans, 1/2 inch.
4. Cut the excess fabric. Pin or tape in place.
5. Select a thread of the same color and thickness as the thread used in the original hem.
6. Sew hem into place. Iron the edge of the new hem on both legs to create a perfect crease.
(First published in Marie Claire, July 2006 as part of "15 Things Every Woman Should Know"; Photo source: sxc.hu)