A cake that looks professionally decorated will have a smooth, clean base for decorations. Uneven, lumpy or crudely frosted cakes will not have that professional quality, no matter how beautiful the design. When covering a cake in fondant, use a piece several inches larger than the cake itself. Smooth the fondant onto a crumb-coated cake and then cut the excess off just below the bottom edge of the cake.
If you're using buttercream, use a large spatula dipped in hot water. Place the cake on a turntable and slowly spin as you hold the spatula steady. Once the sides are smooth, use the spatula to finish the top.