The exercise is the same movement as an ab crunch, but using the weight instead of gravity. According to Legendary Abs, 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. Follow the ordering rules in Question 7. 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 writers 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. Health For Life's Legendary Abs recommends three or four times a week. 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. You can't just remove the fat from that area on its own. Legendary Abs claims that side bends develop the oblique muscles under the fat and therefore make the fat more prominent, but some people feel that the obliques simply can't get big enough to be noticeable.