Eat clean and work all of your core.  Planks, side planks, supermans are all amazing workouts for your core.

Hanging leg rises and resistance crunches

The abdominal muscles are a group 6 muscle. Meaning they respond well to heavy weight. So if you want to build them, use heavy weight and low reps, but focus mainly on your diet. If your body fat is under 12% then your abs can make an appearance. It's a good idea to work your lower abs FIRST in your ruitine because they are stronger and harder to stimulate more than the other muscles. But DO CARDIO more than anything. Try out HIIT which is high intensity interval training. That's the fastest way ;)

For women to have abbs showing you will need to be in a bodyfat range of roughly 15 to 19. Depending on how strong you want them to show and some girls lose fat on their stomach some sooner. Having a strong core is important but the most important aspect is being in the right bodyfat range :-)

I was going to say the same thing as Preston, 8-12% is for males, Im not sure if it applies for women as well though. I know that working with too much body fat would only make your belly bigger though. My suggestion is to do 20-30 min cardio 4-6 times a week and do crunches/russian twists. Good luck!