Wilton's #789 Cake Icer tip is the largest icing tip the company produces. You can frost an entire cake using the #789 tip and many cake decorators prefer to use it instead of a spatula. It can be useful for frosting a smooth layer of icing on your cake or for applying a simple crumb coat under a layer of fondant. This tip is most commonly used on round cakes but can be used on sheet cakes. This tip really helps speed up the process of frosting your cake and also helps ensure an even layer of frosting is applied to all surfaces. It is popular with beginners in addition to people who decorate a lot of cakes.
Since the #789 Wilton cake tip is so large it is only compatible with a 16 inch disposable bag and can only be used without a coupler. A 12 inch bag is too small for this tip which itself is approximately 2.5 inches wide.
A good rule of thumb when using any cake icing tip is to place the tip into the uncut disposable bag, feel where it stops and hold that spot with your fingers. Cut just below where you marked the bag and see how the tip fits. If you didn't cut enough, just snip the bag a little more until just a bit of the tip sticks out. It's always better to cut off small amounts until you get it right instead of cutting off too much at the beginning.
First, make sure the cake is cooled completely before applying the frosting. You may even want to let the cake chill in the refrigerator for at least an hour. This helps prevent crumbs from getting in your frosting. Depending on your preference, you may or may not want to apply a crumb coat before you frost the cake.
Hold the bag at a 45 degree angle and lightly press the tip against the top of the cake in the center. Gently squeeze out the icing in a circular motion as you cover the entire top of the cake in one continuous motion with lifting up the tip. Make sure that the last outer ring of frosting overlaps the top edge of the cake as well as the sides. Now, start at the bottom of the cake and squeeze out icing as you slowly turn the cake. A turntable makes this job much easier. Keep moving up until the entire cake is covered. Then, smooth the top and sides with a spatula using the same methods as before.