A traditional wedding cake has more than one tier. The tiers are either stacked upon the layer below or held up with cake columns. Although this type of cake may appear intimidating, the actual icing process can be done step-by-step, with the icing applied to each individual layer before the tiers are assembled. A few professional cake-decorating tips and some icing tools will help you ice the tiers smoothly.
Prepare a large batch of icing instead of two or three smaller batches to ensure that all the tiers will look the same. This is especially important if you're adding any colour to the icing, since it may be difficult to match the tint exactly if you need more icing than you made originally.
Place your cake tier on a cake stand. A rotating stand, although not essential, makes it much faster to ice the sides of the tier. A typical cake tier has two or more cake layers with filling between each. If the filling is soft, try refrigerating the tier for 30 minutes before icing.
Fit an 18-inch icing bag with a large tip. Half fill the bag with icing. The bag and tip are available at most hobby stores or from a cake decorating supply store. Use a tip with a 1-inch oblong opening.
Hold the icing bag in one hand and rotate the cake tier with the other, holding the icing tip just above the surface of the cake while squeezing out a steady stream of icing. Continue to turn the tier, applying a single layer of icing to the sides and the top of the cake. By doing this, you avoid getting crumbs in the icing.
Smooth the icing on the sides and the top of the tier with the 6-inch taping knife or a similar wide spatula. A regular butter knife is too narrow to create a smooth surface, but a taping knife, traditionally used on drywall, will smooth an entire side by if you hold the edge vertically along the side while turning the cake plate.
Repeat the smoothing process, working quickly back and forth from the top to the sides to create an even top edge. You may have some excess icing to wipe off the knife as you go. Ice each tier in the same fashion before stacking them or adding the cake columns.
Pipe an icing border around the top edge of each tier and around the bottom to conceal the place where the cake meets the plate. Decorate each tier with sugar flowers or other adornments before assembling the cake.