The key with the belly flutter is to get relaxed and practice. It is a contraction of the muscles that control breathing and not actually the abdominal muscles. Usually these muscles are referred to as the diaphragm. Strictly speaking the diaphragm is actually a set of involuntary muscles (meaning you can not control them) and the muscles you are using are called the intercostal muscles. These muscles, just like any other group of muscles, needs to be stretched and exercised to be effectively used.
This is a simple E how article on how to do the basic belly flutter. Below that are two videos. The first is a short explanation on flutters and the second goes in a bit more depth.
If you are practicing the belly flutter I suggest you start laying down or sitting. Doing this will help you relax you abdominal muscles since you do not need to focus on posture or layering. Also laying down forces your intercostal muscles to work more effectively which can help in isolating and practicing this move. The most important thing about this step is to practice, which I plan on doing as much as possible.
Note: How to stretch you breathing muscles
I learn this one in Drum Corps years back. Stand up straight with you hands clasped over your head. Take a deep breath in and hold it. Take two to three more little sips of air. At this point you should feel full to the brim with air. Now hold the air in and slowly do a side bend stretch. You can do this to each side and if you have enough air, you will be able to stretch the muscles that are used for breathing. I find this is good to do beforehand after practicing the belly flutter. (Warning some times taking in that much air can make you dizzy. Its normal. Just resume breathing regularly and it should pass.)