As Arnold Schwartenegger says, "Abs are made in the kitchen, not the gym".

Tone

Build muscle and eat clean

Ab workout... Duh

Diet changes will flatten your stomach. Core exercises too.

Work on posture and stretching to elongate your spine. Bad posture can make someone look bigger than they are. Good posture can flatten your stomach.

cut out foods that cause inflammation, ex- gluten, wheat, soy