Lift heavy weights.
Are you male or female? Age range?
Muscle = dense, fat = not-so dense
Muscle = hard, fat = squishy
Flabby stomach = stomach fat
You can't "tone" your stomach. The only way to get rid of the flabbiness is by getting your BF% lower. You can't spot reduce fat either.
Unless it's flabby because you used to be overweight and now it's just "extra" skin or because you gave birth and it's stretched.
Truthfully, the key to a flat stomach is low body fat.
Strong abdominal muscles can help "sculpt" the stomach and improve the curvature, but the only way to get rid of the rolly-bit around the middle is to burn it off through healthy diet and continuing your exercise routine.
Muscle building exercises help (all around body, not just abdominal) by both consuming calories to perform (leading to fat loss) and increasing your resting metabolic rate by the presence of the new muscle tissue (also aiding in fat loss).
You appear to be within a pretty healthy weight range for a girl your height, so don't go to crazy. Sometimes that little bit of pooch in the stomach just seems impossible to get rid of, in which case you should be happy that you are healthy, in shape, and know that you can be attractive even with a little bit extra on your stomach. I don't think I've ever known anyone who was totally happy with their body appearance, even amazing fit and stunningly attractive men and women both.
My own personal story: when I was younger I was in amazing shape, 6' tall and 170 lbs, ran 5 miles each morning, trained for marathons, lifted weights, crazy low body fat percentage. I still had a tiny spare tire despite everything. I obsessed over it back then. Looking at myself now I would happily trade-in to only have that little bit of flab.