Like any other muscle, you build bigger abs by training and feeding them. The three primary mechanisms that stimulate muscle growth are first- Mechanical Tension, second- Metabolic Stress, and third- Muscle Damage. You should seek to apply each of these to your ab training. Also, because of the anatomy and function of the abs, it's possible to distinctly target the upper and lower abs when training.
Also, when doing ANY exercise (not just abs) make sure to build and concentrate on your mind-muscle connection with the targeted muscle(s). This can be difficult when you are just getting started, but you should be able to feel and contract your muscles. Likewise, make sure that your form is as good as possible when doing an exercise to elicit the maximum benefit.
So what are some examples of what this looks like in practice?
- General Ab Training
- Mechanical Tension Exercise: Cable Crunch / Ab Pulldown
- Metabolic Stress Exercise: Bicycle Crunch
- Muscle Damage Exercise: Ab Wheel Rollout
- Upper Ab Training
- Mechanical Tension Exercise: Weighted Crunch
- Metabolic Stress Exercise: Physioball Crunch
- Muscle Damage Exercise: Ab Roll Up w/eccentric focus.
- Lower Ab Training
- Mechanical Tension Exercise: Hanging Leg Raise
- Metabolic Stress Exercise: Flutter Kicks or Scissors
- Muscle Damage Exercise: Dragon Flag
Note: The goal of each mechanism of muscle growth is as follows; Mechanical Tension seeks to maximize the resistance that a muscle can actively work against. Metabolic Stress seeks to create a pump/burn mid-exercise and to push through that as much as possible. Muscle Damage seeks to overload a muscle primarily through eccentric overload, but it can happen in other ways as well.