Low body fat percentage +ab workouts.
You need to have almost no fat for abs to get really defined and cardio is good for that. Combine that with a workouts and you'll wonder how you got abs so quickly:)
first thing is diet which is 80% of the process and the most important thing, this dose not mean eat very low kilojoules and cut out fats and carbohydrates, you will need to eat the right ammount of KJs for your body weight and your goal, the right ammount of carbs for your exercise and plenty of the right fats i.e low fat milk i know that sounds confusing im talking about getting fats and i say low fat milk but when a product says low fat it means low in trans fats and saturated fat you will still get fats called "plant sterols" from it, other healthy fats would be avocado, natural yoghurt, lean meats and the best one for abs is nuts,
as well as fats you will have to have lots of protien eat high protein meals and snacks before and after gym like high protein lean meats - lamb, turkey, kangaroo and eggs, broccoli and spinach.
once you get the diet going and your sticking to it your 80% of the way to your six pack, and onto the work out which makes up 20% of getting abs and it can be broken down into 15% cardiovascular and 5% lifting weights forget all the sit ups and body weight exercises and make sure you do some serious cardio every day of the week because weight loss is the only thing holding you back from seeing abs hence why diet is key, i would sujest cardio exercises like bike because lifting your knees high and activating your hip flexors will help with getting abs also do plenty of the exercise called mountain climber, while doing cardio everyday of the week plan a 3 day work out regimen and lift heavy weight doing some weighted ab exercises like cable crunches with medium sets 3-4 low reps 6-8 high weight 85-90% of you ORM always have protien when your lifting and ALWAYS have rest days after, in only six weeks you will see real results.
Abs are made in the kitchen. If you didn't know that look it up and understand fully that exercice won't outweigh a bad diet!!