You can do this standing, sitting, or lying down: just pull your stomach in as hard and as far as you can. Then hold it for half a minute or so (and keep breathing, by the way).
Yes that's "all there is" too it. You ensure that your mid-section is tight, so that you form a solid bridge between your toes and hands - don't let yourself buckle down to the floor.
Some of the exercises may seem simple - and indeed in your case, you might find some very easy, especially as you don't carry any excess weight, but they will still help you develop some muscles.
Other ab exercises, such as hanging leg raise might be more challenging for you.
As for Ian King's book - I too have a copy. I prefer Chad Waterbury's workouts, but it's purely a personal choice. Ian has a great approach to exercise management (push/pull). At your stage of development, you will make great progress following any of these workouts. Just make a start, take it easy and worry more about getting your form right than pushing yourself with really challenging weights right now. Once you've got the techniques honed-in, then start going for maximum intensity to meet the exercise repetition targets.
One golden rule that I have learned from Chad Waterbury's writings is to NOT go to muscular failure during a set of repetitions.
Regards and take care,