Modifying the waistband on a pair of jeans may be a way to get a better fit or to get new use out of a pair of tight trousers, but its important to remember that unlike shrinking the waistband, enlarging it requires additional fabric, sewing skills or a trousers stretcher. Unless you previously took in the waist of your jeans with a simple seam that can be removed to enlarge the jeans once again, you need to be prepared for the fact that enlarging the waistband requires noticeable tailoring. Wearing shirts that do not require tucking in keeps your tailoring concealed.
Measure your waist to determine how much you need to expand the waistband. Visit your local fabric store and purchase denim fabric that matches the colour and fade of your jeans. Bring the jeans along to compare, for a perfect match.
Cut the new denim into two, equal and identical, wedge shapes. Make sure the width at the base of the wedge matches half the length you wish to add to the diameter of your waistband plus enough 1/2 inch for seams. The height should match the height of you waistband, with a 1/4 inch of extra space to fold over and sew a seam.
Open the side seams on the waistband, using scissors or a seam ripper. If the waistband does not have seams, cut a straight line down the sides. Avoid cutting past the waistband and into the body of the jeans. Use your measuring tape to make sure your cuts are evenly spaced.
Fold over the right side of the base of each wedge and sew a seam to finish the fabric. Turn your jeans inside out. Position one of the wedges, with the point facing down and the finished edge aligned with the top of your waistband, in one side of the cut. With both raw seams pinched upward and the outside fabric touching, sew the sides together.
Spread the cut of the waistband so that the opposite, unattached side of your fabric wedge lines up with the other side of the cut. With both raw seams pinched upward and the outside fabric touching, sew the sides together. Continue your stitch until you overlap the opposite stitch you made to secure the other side of the wedge to the jeans. Trim excess fabric from the wedge that appears outside your stitching. Repeat on the other side of the waistband, using the other fabric wedge.