To do a basic sit up, lie on the floor with your knees bent and your feet flat on the floor. Cross your hands across your chest and smoothly lift your upper body, from your shoulder blades to your lower back, off of the ground. Sit all the way up, pausing for a moment at the top of your motion. Then, smoothly lower your body back down and repeat. Don't strain or use jerky movements and don't lift with your neck. The stress should be on your ab muscles, not your neck. Keep your back straight - never hunch while doing sit ups. To do a crunch, which is easier than a sit up, just lift your shoulder blades off the ground - not your whole upper body.  Sit ups and crunches work your central abdominal muscles. Your ab muscles, as a whole, run from the pelvis to just below the chest. The transverse muscles are rather large, but the rectus abdominus muscles are fairly thin. For the rectus abdominis to appear beneath the skin, you will need to both build your core and reduce fat around your waistline. For well-rounded, sculpted abs, it's important to hit every part of your abdominal region. Another good core workout is the plank exercise. This exercise, which is a good core workout on its own, is highly versatile - by adding different poses and/or movements to the basic plank, you can work a wide range of muscles. Lay on the floor with your legs straight and pulled together. Keeping your legs straight, slowly and smoothly lift your legs to a vertical position so that they're pointing at the ceiling. Gently lower your legs to the floor and repeat. Don't separate your legs or bend your knees as you do this exercise, unless you find it too difficult - in this case, modify the exercise so that you're bringing your knees to your chest, rather than pointing your feet towards the ceiling. For an extra challenge, try holding a medicine ball or a small weight between your legs as you do this exercise!  Leg lifts work your lower abs. Though they're especially hard to work and "define," well-toned lower abs can take your six pack...

TL;DR in Vietnamese:
Bắt đầu với bài tập gập bụng. Tập nâng chân. Gập bụng đạp xe. Tập chân, mông và lưng với bài tập bước chùng chân. Đừng tin vào những quan điểm sai lầm về việc tập bụng.