Most people just "throw in" a little ab work at the end of their workouts, but to really build your abs you need to put some thought into training them. You'll need low to moderate reps, weighted reps, high frequency, and prioritization.

1 Low to Moderate Reps

Maximizing muscle growth requires lower to moderate reps in the 8-15 range. It's also important to hold the contraction for a one-second count on each rep, regardless of the exercise, to recruit the muscle.

2 Weighted Reps

Whenever possible, add resistance with weight. Most people increase the load on exercises for major muscle groups but they don't on ab exercises. That's a mistake. Think ankle weights for leg raises, or a 45-pound plate for crunches.

3 High Frequency

High frequency training builds abs the best. This means hitting them 4-6 times per week. Four sets per workout is usually adequate. If pressed for time, and you can't feasibly hit a 4-6 day a week frequency, then do a two-exercise superset where the first exercise is predominantly lower-ab focused followed by an upper ab exercise. An example would be hanging leg raises immediately followed by rope crunches.

4 Prioritization

I'll often alternate between lower and upper ab work with a two-thirds to one-third ratio, respectively. My upper abs always get taxed even when focusing on lower abs, whereas exercises targeting upper abs do very little for activating lower abs. Therefore, prioritize lower over upper ab work.