Begin with a frosted cake. It doesn't have to be perfectly smooth. Clean off your icing spatula.
Beginning at the bottom of the cake, turn your spatula so that it is horizontal and press it against the cake. Gently spin the turntable, keeping pressure against the side of the cake. Once you have turned the cake 360°, you should see a smooth imprint around the bottom of the cake. However if it isn't as smooth as you'd like, you can spin it again, keeping the icing spatula pressed against the same spot.
Scrape off any excess frosting that's on your spatula. Move the spatula directly above the first imprint and repeat the above steps.
Continue to move up the cake, scraping off excess frosting as necessary, until you get to the top of the cake.
Continue around the top of the cake, moving inwards. Serve as-is, or add more decorations as you'd like!