This is the most fun step, because it's basically an excuse to eat a ton of cake. Try a few recipes for each flavor of cake, filling, and frosting. If a recipe feels fussy or requires a ton of a very expensive ingredient, just scrap it. This is not the time to make a cake that requires lots of delicate folding and perfect timing. Pick a cake recipe that's foolproof and simple and you'll have one less thing to worry about. As it is, homemade cake tastes so much better than what caterers provide that it's basically guaranteed to be yummy. Mix and match cakes, fillings, and frostings to find what combination is both tasty and easy.
If the wedding couple is excited for a homemade cake, hooray! Ask them if they have wedding colors and what styles they like. You can look at photos of cakes online together or have them send you a few photos for inspiration. Make sure you understand their vision of their reception so your cake fits in with the rest of the décor. Keep in touch throughout the process, but don't badger them with questions — a cake is just one of many things they have to plan.
You should use a recipe specifically designed to be a wedding cake as it will ensure that it is sturdy enough and that it makes the right amount of batter and icing. For a three-tier cake, you need to make three cakes of 12", 9" and 6", as well as a massive amount of icing. Most three-tier cake recipes have a step-by-step schedule for when you will need to bake and assemble each part.