How to build 8 pack abs?

Leg and knee raises

Do leg and knee raises for the lower abs and the only way you are going to burn up the fat from your abs is by doing a 15 to 20 minute high intensity cardio workout every alternate day and just do your upper abs and lower abs on two alternate days as well and 1000 sit ups is just going to build hard abs but they would not show until you burn the fat around your mid-section and that can be achieved only by a low carbohydrate, low fat diet along with the cardio.

Roman chairs

Also you can do Roman chairs with weighted plates. First do two sets each of 20 reps and then take a weighted plate and do. The plate should be so heavy that you could do only 12-15 reps. Do 3 sets of this. On this day only, train the lower abs also.

After two days or so, in the same week, train the oblique muscles. Do heavy dumbbell side bends and then side crunches with ball or so. After that, do barbell side bends or twisting.

Train abs twice a week

One day abdominus rectus and other day obliques. If your abdominal muscles are really growing, you will feel your tight abs even though, you have a layer of fat outside since spot reduction is not possible, so if you want to remove that thick layer of fat, you need to do some cardio also as suggested above.

Go for heavy weightlifting for large muscle groups

Especially those "press" exercises like shoulder press, bench press, leg press etc. should be done heavy. Heavy is a relative term and not absolute. By heavy, I mean that the weight, which will not allow you to do, more than 8 reps. Do pyramid sets. Don't do cardio more than thrice a week. Also include some proper weight lifting exercises like clean and press, dead lift. They also work for overall thickness of the body.

Burn more calories

Also note that since you need mass, so make sure you are not burning more calories than you eat. Eat six to seven times a day. Taking two big heavy meals is not the idea. I mean you should divide your meals to six times a day. The meal should contain 60% carbohydrates, 20% protein, and 20% fat. This doesn't mean that you should calculate the calories, etc. every time. I just mean that you should get the rough idea and include lots of compound exercises instead of isolation ones. For each muscle group, don't do more than 4-5 variations and don't train for more than one hour.