You just put a thick coating of buttercream on the cake, and then drag the back of a spoon through the frosting as you spin the cake on a turntable.
Not a huge problem, but to me smoother is better. Like everything in life it is a delicate balancing act - getting the buttercream soft enough that it is smooth and silky but not too soft that it starts to droop and slide off the cake (been there done that).
But I have found that if you put enough pretty gum paste flower on the cake, no one notices that the buttercream is not quite right.
Happy Decorating,
Carol