Place a cake with the cardboard round on a cake stand (or if you don't have a cake stand on a cutting board will work.)
Using a serrated knife, such as a bread knife that is long, determine the middle of the cake on the side.
Begin to slice the cake at the side. As you slice toward the center of the cake, turn the cake as you slice. This will allow the knife to stay on the same path using the area of the cake that you have sliced as a guide. (see video)
Once both cakes have been split, place one cake on the cake stand or cutting board with a cardboard round underneath.
Remove the top piece of the cake onto a second cardboard round and set aside.
Layer icing on the first layer of cake and spread to cover about 1/4" thick.
Place the next cake on top by sliding it off the cardboard round on to the icing.
Repeat the process of layering icing and cake until you place the last cake on top.
Once you have all 4 halves of the cake layered with icing begin to place a crumb coat of icing onto the top and sides of the cake. (see video)
Once a thin coat is placed on the cake, refrigerate the cake to stiffen up the crumb coat.
Remove the cake from the refrigerator and continue the process of layering the final coat of icing on the top and sides.
Using a bench knife and a long offset spatula, smooth the icing on the top and sides. (see video)
Once the desired look is achieved, decorate the cake sides with walnuts halves or pieces.
Decorate the top as you wish, additional walnuts or get creative piping icing.
Chill the cake makes it easier to slice because the cake and icing hardens up or you can serve immediately. Enjoy!