The first thing that should be done when icing a cake with buttercream frosting is to wrap the cake in plastic wrap and chill it in the freezer. This will ensure that when you trim the top of the cake there will not be as many crumbs. After triming the top of the cake to make it even, turn the cake over so that the bottom of the cake is now the top. Now take a piping bag filled with buttercream frosting fitted with a large basket weave tip and pipe horizontally going around the entire cake.
Continue doing this above where you previously piped in order to cover the entire side. Now put a big dallop of buttercream on the top. Taking a large offset spatula, start to smooth the top down. Now smooth the sides down, scraping the excess frosting off as you work. Using a dough scraper, smooth down the sides getting them really smooth. Use your spatula to smooth out the top again. Refrigerate the cake until chilled and decorate how you wish.