Build big abs the same way you build any muscles. Excercise them and increase the resistance over time. Oh also keep the body fat down. Nothing kills ab definition like stomach fat.
Buy a 25 and 45 lbs. dumbells and do negative incline situps everyother day and high rep general core. Move up to 45 and more reps overtime.
Oh and forget having 300 abs or TO abs. Those guys use PEDS and they work, bigtime!