How to Take in a Jeans Waist by @Glitter+Wit
Step Two: Rip
Clip each line of waistband stitching a half inch or so outside of your markings. Jeans are usually sewn with a chain stitch, so be sure to clip the beginning and end of each stitchline you wish to remove. Trust me, pulling out chain stitching is really fun, but you don’t want to get too carried away! Go ahead and remove that section of stitching now.
Remove belt loop(s) at center back. With the interior chain stitches removed, you can clip away the corresponding thread on the outside of the waistband.
Separate the outer and inner waistband by clipping the stitches at the top of the waistband until you reach your intact chain stitches.
Measure the distance between your waistband markings. Memorize it (or just write it down I guess).
Clip the chain stitching an inch or so below the end of your center back seam markings. Pull from the top to remove chain stitching, and remove corresponding exterior thread.