Step the first: procure cake. This set pertains to a square cake; the slightly simpler modifications for creating a round cake will be explained in text.
Ensure that your cake is frozen completely solid enough to handle easily, relatively flat on top, completely clean of any baking pans or sheets except for whatever it is sitting on but not stuck to, and not broken or crumbling. If the cake is any of those things, get another cake and try again. This isn't a baking guide.
Using a piping bag equipped with a Base Icing Tip and filled with frosting, pipe lines long-ways onto the top of the cake, with the textured side of the frosting ribbon facing down. Use as many lines as you need to in order to ensure the top of it is covered; this guide is pictured with four.
If your cake is round, concentric circles that cover the top are preferred.
Using a straight-edged scraper, flatten the layer of icing. It should be about a quarter of an inch thick.
Place the second layer top-down onto the first layer, taking care to center it. Top-down is to ensure even corners on top.
Ensure that the cake is centered on the board, the board is centered on the turntable, and the sides are as square as possible up-and-down. A fluffy cake may need some additional frosting between the layers around the edges or any low spots.
If working with half-sheet cakes in a commercial setting, this is the time to cut them down to quarter and eighth sheets, and distribute onto their own boards.
With the piping bag and base icer tip, cover the cake in a layer of frosting. The top should be done in the same way as the middle. The sides on a round cake may be one continuous line. On a square cake, ensure extra frosting is on the corners; use a new stroke for each side. Use one pass of the piping bag per layer of cake at minimum.