When the elastic in your jacket cuffs or waistband is too tight, you do not have to settle for being uncomfortable. Adjusting the elastic can turn an uncomfortable jacket into a nice-fitting garment. If you are adding elastic, get elastic the same width as the jacket's existing elastic. You should be able to feel through the elastic's casing by pinching the casing with your fingertips. Line up a ruler with the elastic's edges to get a measurement.
Pick a spot in the center of the casing and use the ruler to measure out 2 inches each way from the center. Place a safety pin through the elastic and the casing, so the elastic is pinned to the fabric in both spots.
Cut every second stitch between the safety pins by sliding the tip of the seam ripper under the individual stitches, pushing forward and lifting up. Pull the casing open and use your fingertips to pick the loose threads out of the casing.
Tug gently on the elastic to make sure the safety pins are holding it in place. Use the scissors to cut the elastic in half in the center. If you are tightening the elastic, overlap the ends as much as needed. Use the sewing machine to sew over the overlap at least four times to make a zigzag stitch. If you are adding length, use the scissors to cut a strip the needed length. Overlap the ends of the strip and the ends of the original elastic by 1/2 inch; zigzag back and forth over the overlaps at least four times.
Tuck the elastic back into the casing and remove the safety pins. Refold the raw edge of the casing and put it back in its original position. Set the sewing machine to make a straight stitch. Sew the casing, hitting the original stitching line with the needle.
Clip the thread ends off with the scissors.