Preparing a perfectly smooth iced cake, while keeping crumbs out of the icing can seem like a tricky thing–but over the years we've developed some methods to make it easier. Start by positioning the cake on its serving board on the turntable. Remember to thin the icing with 3-4 Tablespoons of corn syrup so it spreads easier. The trick to keeping crumbs out of your icing is to glide your spatula over the icing, never allowing the spatula to touch the cake surface or to pull already spread icing from the cake surface.
Trim a 16 in. Featherweight bag to fit tip 789. Fill bag half full with icing. Hold bag at 45° angle and lightly press tip against cake. Squeeze a ribbon of icing in a continuous spiral motion to cover cake top, with last ribbon forcing icing over edge of cake top.
To ice the sides, squeeze icing as you turn the cake slowly. Repeat the process until the entire cake side is covered.
Rotate the cake slightly and repeat the procedure, starting from a new point on the rim until you have covered the entire top surface. Smooth the center of the cake by leveling the icing with the edge of your spatula. For easier smoothing, it may help to dip the spatula into hot water, wipe dry and glide it across the entire surface. Set the cake aside and allow the icing to crust over for at least 15 minutes before decorating. At that point you may also lay Non-Stick Parchment Paper on the iced cake top and gently smooth with the palm of your hand.