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.