Place cake on a cake turn-table, and ice the rest. The best way to ice a cake is to start from the top, and then move on to the sides (in my opinion). Put on a lot of icing at first, and then use your flat spatula to scrape away as you turn the cake turn-table for a smooth finish. A great video on cake icing can be found
here. The sides of this cake don't need to be perfectly flat, because you are going to cover them with chocolate swirl rods anyways.