The first step to making a decorated cake, is to bake a cake. You can make, for example, 3 layer yellow cake from scratch or a box. Bake and cool your cake. Wrap in plastic and cool the cake for minimum 1 hour, better 2.
Make sure you cut off the edges before applying frosting and crumb coating. It is importsant that layer cakes are the same size, or conical, if you go for that look.
Prepare your buttercream: Mix the soft butter and shortening on medium speed until smooth and blended. Gradually add in the powdered sugar, meringue, and vanilla. Add the milk as needed to smooth the frosting and thin it your desired consistency.
Now add a bit of frosting to your cake board and place your first layer of cake down. This will secure the cake to the board.
Add a thick layer of frosting and smooth it over, using a large piping bag and a cake icer tip to add the frosting. Add the second layer and repeat.
If you did not blind bake or cut your cake layers, place the top cake layer on upside down so that the top of the cake is flat.
Add a thick layer of frosting to the top of the cake and push it over the edges.
Press the frosting into the sides of the cake and then smooth out the top and sides. Having a turntable cake stand makes this task a lot easier.
Make it as smooth as you can. This buttercream will crust over a bit making it easy to use a paper towel to smooth out any edges later.
Make sure, the cake is fully crumb coated and ready for the next step! Good job, you!