Turn the jeans inside out
Put the jeans on and pin the excess fabric at the back waistband
Remove the jeans and mark the new waistline with chalk or pins
Open the waistband at the back center or side seams
Remove the waistband stitching in the area to be altered
Take in the back seam or side seams evenly
Sew a new seam along the marked line
Trim excess fabric if needed
Reattach the waistband
Topstitch the waistband closed
Try the jeans on to check the fit
Repeat adjustments if necessary