Lots of calorie burning cardio,
Some good ab work: plank, bicycle crunches, Russian twist, leg lift...
Calorie counting to lose weight, use myfitnesspal or noom. Eat mostly veggies, fruits, some whole grains like brown rice or quinoa, some nuts, some meats, drink only water (or black coffee /unsweetened tea) basically simple real whole foods and water.
Most of the challenge of getting abs is getting the low body fat so you can see those muscles. That means cardio and calorie counting. Of course do abs exercises too.