There's no better way to improve your ability to ice masterpieces of cupcakes and cakes than to simply practice. And the tip presented in this cake decorating video is indispensible. Using a #21 tip (called an open star tip), you'll learn how to ice a shell border.
A shell border design looks like a downturned shell with striped indents all over. Frequently used to adorn cakes and cupcakes, the shell border is great because you can use it to cover large border areas and the method is rather easy to learn. Watch the video for the how-to.