1. You lower your body fat percentage to a point where your abs are visible. This is done through proper nutrition and general exercise. Unfortunately, most people think its all about stage 2, but in reality for most people the body far percentage is the most important by far.
2. You work out your abs. How much and how intensively depends a lot on what else you do. E.g. people who engage in other sports will require less work than a couch potato.
Use a variety of exercises targeting the upper and lower abs as well as the obliques. Treat it like any other muscle and give it time to recuperate, i.e. do not train it every day (2-3 times a week is good). If your reps get very high, add weight (e.g. weighted crunches).
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