The only way to get better at doing exercises is to actually do them and watch yourself progressing. I take a yoga/pilates class to strengthen my core. It's a laugh. The instructor keeps thinking I will fall down, and I certainly can't do the exercises as well as others. BUT I noticed after the last round of classes, that they do work, and gave me spin-off benefits such as better posture. You can only go to the extent that you can for that day and reach for more the next.
Leg lifts and sit-ups work your ab muscles, but your core is made up of many more, and you need exercises to work all around your core. Look for exercises that work your abs, obliques, erector spinae, hips, glutes for an all-round core workout.
Do or don't do; there is no try (Yoda)