To smooth the sides, rotate the turntable while you rest the spatula lightly against the cake, perpendicular to the turntable.
You will notice a ridge or seam where the sides of the frosting meet the top of the cake. If you were to frost first the top and then the sides, this seam would remain, but frosting the top last makes it possible to eliminate it.
Beginning at the seam, hold the spatula flat, and draw it from this outer edge of the cake to the center, using a light stroke. Now rotate the cake on its turntable an eighth of a turn, and repeat this technique with the spatula. Continue until the seam has disappeared and the top is smooth.