In this article, I'll try to keep it short as there is nothing complicated about this topic at all.
As a natural lifter, you only need to apply the way you progressively overload your other muscles to your abs. Abs are muscles, and muscles grow when stretched. Therefore, it must be logical to conclude that abs will grow if we progressively overload the stretching component of the muscle using a mixture of reps and weight.
It's as simple as that. For this article, I won't even touch on machine work for abs because I don't use them. I have never tried a machine (mostly because I don't have access to one) because ab work is hard enough when done properly.
I'm going to list 3 movements you can do that I guarantee will be enough to get all of the hypertrophic abdominal development you could ever want.
Sit-ups with Decline Bench or with Weight or Both
When done properly, sit-ups are probably the best movement you can do. All you need to do them is either something heavy to put your feet under to anchor you in place, or a decline bench.
Here is how I like to do them:
1 - If done without equipment, sit down in front of something heavy that you can slide your feet under. Make sure your legs are straight in front of you, no bending at the knee. Now slowly lower your upper body to the floor and be sure to take your sweet time on the negatives. How fast you get up from the movement is up to you, it doesn't really matter. Instead, focus on getting high quality negatives out of this movement.
2 - If done with a decline bench, set the bench to the lowest decline (like -5 to -10 degrees), and do sit-ups like normal, but instead of touching the ground, your back should go down to about parallel to the floor, and then go back up. Go slow on the way down, not only to milk the negative stretching proportion of the movement, but to keep your back safe as this movement can easily cause lower back strain if you don't control it. Just be smart about it and don't bounce up and down like an idiot.
3 - With weights, either of the two movements could be done. I like to grab a weight plate (start small at first because this will make the move exponentially harder) and hold it behind my head if I'm on a decline bench, or with my arms straight up above my head if I'm doing this without a decline bench.
Leg Raises / Knee Raises
These will have more of a focus on the lower half of your abdominal muscles. You can do these lying down on the floor (and weighted), hanging from a bar, or on something like a dip station. If you can lie down or hang around, you can do these. Focus on controlling the movement, especially on the way down. I would say never let your legs hang completely down if you can, this keeps tension on the abs and wears them down quicker.
Russian Twists
These are great for obliques and can be done anywhere you can perform a sit-up. Simply lower yourself halfway through the range of motion for the sit-up, hold yourself there, and put your hands together. Move your hands from one side of your body to the other, slightly twisting your upper body doing so. You can weight these by holding a weight plate in your hands while doing the movement.
Make sure you train your obliques, a great painting should be displayed in a grand frame.
That's all!
Building abs is simple. Getting shredded is not the way to get abs. If you follow that logic, you might as well not lift at all and starve yourself because if your body fat percentage is low enough, we can see every muscle.
Just build your abs like any other muscle and they will show through even at higher bodyfat percentages (up to 18-20 percent bodyfat on the higher end).