If you are looking to create an iOS 7 app and you want your icons to closely match that of Apple’s iOS 7 icons, you follow the grid template.
This ensures that your app icons follow some of the continuity of the iOS apps. It’s also worth taking a look at this post. You can learn a bit about the color palette that Apple used for iOS 7.
It’s worth taking a closer look at some of the iOS icons and learning how they are designed. Use that as a bit of an idea for how to build your own icon.
You can read more about the templates at Design You Trust.