Size up your cake before frosting
Before assembling your cake, look at the layers to judge the order in which they're to be placed. If all went well, the layers should be even. Occasionally, though, odd-shaped layers can result from oven racks that aren't level or from an oven resting on an uneven floor. If your layers still aren't flat, level the tops by slicing off the uneven portion with a long serrated knife.
When you're ready to frost the cake, always choose the tallest and most level layer for the bottom. If the bottom is lopsided, the remaining layers will tilt or slide. For a two-layer cake, start with the bottom layer placed top side down. After applying frosting across the surface, position the top layer right side up so that two level surfaces meet in the middle.
For a three-layer cake, position the bottom and middle layers top side down and the third layer top side up. If the layers are uneven, place the thinner side over the thicker side of the cake beneath.
When a cake layer must be split into multiple layers, always place the crumb or cut side of the layer in the middle, never at the top or bottom. This way, no crumbs are visible on the outside of the cake, and the surface to be frosted will be smooth and even.