If you want to get more practice with your cake decorating you may want to use things called cake dummies. They can be purchased as a 2 in or 3 in thickness. Cake Dummies come in the form of square, round, heart, petal as well as sheet cakes in addition to many other shapes that are available.
There are many places online that you can purchase cake dummies and they can be quite pricey at some of the different online stores. There is a website called Dallasfoem.com and they have the best prices online.
You can use Butter-Cream and Fondant on cake dummies. Treat them as if they are a real cake when you cover them with the icing that you plan to use. Cake dummies are usually used for a certain cake designs in order to get it right before you start the real thing. In order to get more use out of them other than that one time is to cover the cake dummies in saran/plastic wrap and tape it together on the top and the bottom so that when you take the cake apart you just have to pull the saran/plastic wrap off the dummy and wallla, you have a clean cake dummy once again.
If you are going to be using fondant on your cake dummies all you do is cover it with butter-cream just like you would a real cake and then put your fondant over it and cover.
If you plan to re-use your cake dummies in order to get more use out of them, after you have done your practice cake just rise the dummy under warm water after you have removed all of the icing or fondant and then let air dry.
Doing dummy cakes and decorating them is a great way to make several different looks in order to create a photo portfolio for your cake business if that is what you are planning to do. So whenever you create a dummy cake that turns out great take a picture of it and then add it to your cake portfolio for your cake decorating business in order to show what you can do.