You won't just pick up a cast net and throw it perfectly the first time. It takes practice to learn to get the net to fully open. After you've mastered that, you'll need to perfect your aim a bit. It's best to start learning with a small cast net and work your way up to larger nets. I suggest using a five-foot cast net to start out with. This video shows you how to throw a small cast net:
After mastering a small castnet, you can move on to a larger net. Obviously, a larger net has more fish-catching properties because it covers more area. The best thing to do is to practice with the net untill you discover a style that you're the most comfortable with. This won't take as long as you think!
Watch this video of cast netting for sheepshead:
If survival depended on fishing, I'd rather have a cast net than a rod and reel or fishing pole. Why? Because it doesn't require bait, it's very portable, and you can catch more than one fish at a time with a cast net. You can also catch shrimp and crabs with cast nets!