1. Before you begin, be sure that your cake is covered with a layer of buttercream at least 1/4-inch thick. This frosting layer helps the fondant stick to the cake and smooths out any bumps or imperfections on the cake surface, so the fondant layer looks clean and smooth.
2. Dust your work station lightly with corn starch, and begin by kneading the fondant like bread dough until it is smooth and pliable. Fondant, especially if it has been sitting unused for a while, has a tendency to get stiff and crumbly, so you must knead it until it is soft and supple. If it still feels very dry, you can knead in a small spoonful of vegetable shortening to soften it up. By the end of your kneading, you shouldn’t have any cracks along the edges of the fondant ball.
3. Now it’s time to roll the fondant out. If you have a large nonstick mat or cutting board without a texture, those are good options. Otherwise, just use a surface that’s large enough and that won’t mar the fondant with too much texture or patterns. If you have a nonstick rolling pin, use that as well. Making sure your workstation still has a light dusting of corn starch, begin to roll the fondant out in the approximate shape of your cake: a circle for a round cake, a rectangle for a rectangular cake, etc.
4. As I’m rolling, I like to occasionally life the fondant and rotate it 45 degrees. This ensures it’s not sticking to the mat, and it also helps me roll more evenly. Roll the fondant until it is the size you need. For a round 9-inch cake that’s 3 inches high, you’ll need your circle to be at least 15 inches around (two 3-inch sides plus 9 inches across) but you should aim for an inch or two of flexibility on each side. So for your 9-inch cake, the optimal size will be a fondant circle about 17 inches across, with a thickness of about 1/8 inch.
5. Once your fondant is the right shape and thickness, place the rolling pin on the farthest edge of the fondant and roll it toward you, rolling the fondant up around the pin. Quickly lift the rolling pin up and place it on the side of the cake. Start unrolling the fondant from the pin until it hangs just below the bottom edge of the cake, and continue unrolling the fondant across the top of the cake to the other side. When you finish, your cake should be draped in an even covering of fondant.
6. Use your hand to carefully smooth the top of the cake and remove any air bubbles from under the fondant. Be sure that you don’t have any jewelry on that might leave marks, and avoid using your fingernails, as fondant is easily marred and scratches are very hard to repair.
7. Now position your other hand on the side of the cake toward the top and press the fondant into the side. Use your hands together to press toward each other, forming the corner of the cake.
8. At this point, you probably have a lot of excess fondant toward the bottom that will form ruffles and seams if you’re not careful. Start to gently pull the fondant out, spreading the excess fondant, as you slide one hand down the side and smooth it out in a downward motion. Turn the cake and repeat this process again and again. You may have to make several passes over the cake, very gently pulling the excess fondant out and down, and then using your hand to smooth the fondant onto the cake. This is the key to smooth sides, but it is a tricky motion and one that requires some practice.
9. Now your cake should be covered in fondant. If you notice any air bubbles, use a small sharp pin to prick a hole in the bubble and smooth it out. Use a sharp paring knife to trim the excess fondant from the bottom of the cake.
10. Your cake can now be finished with additional fondant or buttercream decorations and borders. If possible you should avoid refrigerating your cake, as fondant readily absorbs moisture, and is often sticky and wet when removed from a refrigerator. Optimally, you should store your cake in a cool location in a cake box, and eat it within a day.