Answer: Start being in a Caloric deficit
Explanation:
While working out your abs through crunches or situps is helpful in terms of making your abdominals muscles look more full and solid, it is not a necessity.
Everyone is born with abs, it is just a matter of maintaining a certain level of body fat percentage that allows them to be visible. (Which may vary slightly from person to person as everyone's fat distribution is unique).
The phrase abs are made in the kitchen should help enlighten your inquiry.
Technically speaking, you do not have to work out a day in your life (although it does help) and you can still maintain your visible abs as long as you are in a caloric deficit, which essentially means you are eating less than your maintenance number of calories.
Some tips that can help you with this process of cutting down would be eating a lot watermelon as a typical watermelon slice is only 85 calories, and being as such watermelon is over 90% water, it is very filling. You should also consider drinking a bottle of water before meals, as your stomach will be filled up more quickly.