The exercise is the same movement as an ab crunch, but using the weight instead of gravity. You should exercise the lower abs before the upper abs and do any twisting upper ab movements before straight upper ab ones. Twisting exercises work the obliques as well as the upper abs. Try to do sets in the 15-30 rep range. Follow the order above Pick easy exercises to start with and when you can happily do about 2 sets in a row of an exercise, try harder ones.
Some experts recommend doing abs at every workout.
Others recommend doing them however often you do anything else in other words treating them as you would any other body part. Since most people want good abdominal tone more than freaky abdominal size, it probably makes sense to exercise the abs with lower intensity and more frequently, rather than with high intensity and less frequently. Thanks for asking. Hyperextensions are a good lower back exercise. Dead lifts, both straight and bent-legged give the lower back a lot of exercise, so if you do them you don't need to add anything else.