If you're not into spending money to have a cake decorated for you, this tip from America's Test Kitchen will help. Trace out the design or lettering you want on your cake with a toothpick before you start piping. You can see if you have room, and you can always smooth it over and try again if you screw up.
If you've ever experienced that sinking feeling when you're icing the top of a cake and realise you're out of space three letters before the end of the person's name, this tip is a lifesaver. The video above shows you how it works. Just grab a toothpick and use it to draw in the frosting what you want the top of the cake to read. Then, if it works, you can use that outline as a guide to pipe your icing. If it doesn't, smooth it over with a knife and try again.
If writing isn't your thing, try using a paper towel for a smooth look. If the cake is for kids, use a colouring book page. Of course, you could always hit the supermarket and have someone do all of this for you, but there's something special about a home-baked cake, and something even more special about decorating it yourself for the person or event it's destined for. This way, you can make sure it's well received, and no photos of it end up on Cake Wrecks.
How to Eliminate Cake Decorating Mistakes [America's Test Kitchen]