Women in the USMC have a bit of a problem... the height weight allowances are more strict than the Met Ballet. It's a problem because muscle density creates extra weight. One of the "cheats" to bypass the weight standards was waist measurement (the other was body fat %). It's a lot easier to have a tape measure around your waist then to jump in a tank of water while everyone snickers up their sleeves at you. Hence... most women marines become good friends with broomsticks.
A person actually has TWO sets of abdominal muscles (okay, my fellow geeks, yes 3 if we want to count smooth muscle, lay off, I'm not counting it). Interior abdominals and exterior abdominals. The ones most people think of with abs are you exterior ones. The sexy six pack ones. These are not the muscles we're going to be working with. ((Crunches, pushups, pullups work those)).
Take a broomstick (sans the bristly bit, they twist off or you can buy a dedicated broomstick from a hardware store). Broomstick in hand go get one of your favorite movies and pop it into the DVD player (I know... hard work). Push play. Stand back far enough that your mum won't say you're damaging your eyes / at least far enough back that if you tried to kick the TV you'd miss.
Put the broomstick over your shoulders behind your head. Drape your arms over it (like you're about to be fetching water). Stand with your feet far enough apart that you won't fall over, and twist. Back and forth. While watching your show. Don't put a lot of effort into it, and don't spin so far you get dizzy. Just twist back and forth. Work your way up to several thousand a day (aka the whole durn movie).
In the weightlifting world and dieting world "twists" got bunked over a decade or more ago. Because they do NOT make your exterior abs pretty. They do absolutely NOTHING for them. They also don't help significantly with weightloss. What they DO do is tighten the INTERIOR abs that hold your organs in place. Which has a rather remarkable effect : you lose INCHES off of your waist. Most people lose 6-9 inches over apx 6 months.
WHY THE BROOMSTICK IS IMPORTANT: If you don't use a broomstick a few things happen. Worst, you're using the wrong muscles. You end up using your back and exterior abs and shoulder muscles (to keep your arms up). One also gets dizzy/sweaty, so one doesn't do the several hundred per day... but more importantly... wrong muscles!. If you want your interior abs to tighten up, do it the "easy" way by throwing the broomstick over your shoulders, standing up straight (hard not to do when a piece of birch pokes you if you slouch), and relaxedly twist back and forth and back and forth. Minimum about 1000. Aka an hour (don't count unless you're OCD and enjoy that sorta thing, just know it's about 1000 per hour).
DO NOT expect to lose inches per week, like pounds per week (heck, I can lose 12lbs per day if I'm willing to screw over my metabolism and body... super unhealthy). One can lose an inch or more per week, especially in the beginning when those muscles haven't been used in ages and ages. But it's quite rare. ALSO the better shape you're in, the less results you will see. But in general (purely from observation... nothing scientific) an inch or two per month max expectation.
ALSO... to help the process along... sit up straight. Pull old world charm out and NEVER lean against anything. Not the back of a chair, couch, wall. Actually forcing your stomach and back to work when you're not does wonders for over all skeletal placement & small muscle strength. All of which = a tighter / more fit "core". Not sexy muscles, but sexy silhouette.