Step One: Frost Your Cake
Frost your cake with buttercream frosting. Place it in the fridge and allow it to chill for at least an hour.
Step Two: Prepare the Stencils
Decide on what stencils you are going to use on your cake and wash them before using it. You may need to trim the stencils down with scissors, so they fit on the cake. Then take your stencil and lightly coat it with a layer of shortening, as this will allow it to stick to the cake.
Step Three: Place the Stencil on the Cake and Add Buttercream Frosting
Once your buttercream frosted cake has been chilled, take it out of the fridge. You may place a piece of wax or parchment paper on your cake with a thin layer of shortening on the cake side to avoid smearing, and give you a place to rest your hand.
Take your stencil and place it on your cake, shortening side down. Take a piping bag of frosting and carefully fill in the stencil with your desired color of buttercream. You may also use a frosting spatula to gently smooth out the frosting.
Step Four: Remove the Stencil
Carefully remove the stencil from the cake. If any of the stencils smeared on the cake, carefully take a knife to remove it. Wipe the excess frosting off of the stencil. Repeat until you have reached your desired design on the cake.
You may use just a cake spatula when frosting the stencil. Just be careful not to accidentally spread the frosting underneath the stencil while filling it in. Using the shortening will act as a glue and help keep your stencil in place, so make sure to put some on to lessen the chance of the stencil slipping.