Making a large cake can be a daunting prospect for a home baker. In practice, the actual baking is not that much more difficult that making any other cake, though the logistics do require some forethought and planning. It helps to bake your layers ahead of time and freeze them, so you have the luxury of just assembling and decorating your cake on the appointed day. Most varieties of cake freeze well, and may even be improved. You might also need to purchase or rent the largest sheet pans that will fit in your oven.
Begin by calculating how big a cake you need to bake. To do this, multiply the number of people you're expecting by the size of portion you expect to serve. In most cases a portion 2 inches by 3 inches is considered appropriate, which would provide 72 slices from a commercial-sized sheet pan.
Decide which pan is the largest you can fit in your oven. At 25 in. by 18 in., a commercial full-sheet pan is too wide for most home ovens, but half-sheet pans will fit easily. If you already have a large quantity of 9-by-13 inch pans, you might prefer to just use those and bake more batches.
Calculate how much batter you will need at one time. If your oven will accommodate full-sized pans, for example, you will need 28 cups of batter for each pan. Check the yield of your favourite recipe, and look up the volume of that pan on a conversion chart. Multiply the recipe enough times to make the correct amount of batter for that pan.
Plan your shopping. Multiply the amounts of each ingredient by the number of batches you're making, and calculate how much to buy. If you're buying in pounds but measuring in cups, use a kitchen scale to weigh the cups. You won't want to run out of sugar in the middle of mixing batter.
Get all the help you can find. One or two friends can be a big help, if only with the lifting and cleaning. If you can, arrange the use of additional ovens during the baking stage. It will save you a lot of time and effort.
Bake your cakes in advance, if you can, and freeze them. If you don't have the luxury of time, allow each cake to cool completely and wrap it carefully in plastic film wrap.
Clear a large enough space to assemble your cake. If you have the option of putting it together on the table it will be served from, this is ideal. Otherwise, you will need a rigid platform large enough to carry the cake but small enough to fit into your delivery vehicle.
Level the individual cakes, if you wish, to make a more finished and professional appearance. Otherwise, use a long serrated knife to trim a thin strip from the edge of each cake where they meet. Slide them together, to make a continuous surface.
Use the end of a cake decorator's spatula to apply a portion of icing on the side of the cake at one corner. Using a rocking motion, spread the icing sideways in an even layer until you run out of icing. Continue, until the entire edge of the cake is covered.
Scoop a large portion of icing into the middle of the cake. Using a rocking motion, spread it evenly with your cake decorator's spatula. If you raise any crumbs, pick them out carefully with a small knife.
Place more icing a few inches away from the edge of the original portion, and spread this around as well until it makes an even, continuous surface with the first portion. Continue, until you've reached the sides of the cake.
Smooth the edges and corners of the cake with your spatula. Decorate the cake as desired with icing, candles or stick-on decorations. Cut the cake as needed, to give the correct number of slices.
If you're intimidated by the idea of making a huge cake from scratch, don't feel badly about using a commercial cake mix. Many commercial bakers and cake decorators use mixes, to ensure a consistent product. You can save table space by stacking a smaller cake on top of the larger sheet, to make a two-tiered design.