Even if you see tiny imperfections in your cake, I promise you that nobody else will. The bride and groom will love their unique gift, and their wedding guests are going to be super impressed by the fact that you baked an entire wedding cake and by how delicious it tastes. No one will notice or care if one tiny part of the fondant is rumpled or your flowers turned out a brighter yellow than you expected.
Sadly, your cake isn't going to be preserved in a wedding cake museum for all time — or maybe not so sadly since that would be such a big waste of butter — so be sure to take lots and lots of photos. Most wedding photographers will take lots of shots of the cake, too, and you can ask the bride or groom for those later. And make sure to get one of yourself with the cake!
To cover the edge of the board, roll some fondant into a long sausage, and roll this out into a flat strip. Trim one long edge with a sharp knife to give a crisp edge. Dust the icing with cornflour and roll it up. Brush the board with a little water and unroll the icing strip around it, pushing it up to meet the bottom edge of the cake. Cut the strip when it meets the first end, and smooth out the join with your fingertips. Using you hand or the cake smoother, smooth out the icing on the board, Use a sharp knife to trim the excess from the edge of the board. Smooth down the edge with your finger tips.
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.