Start by placing a cooled cake (in true fat-kid fashion, I used two) upside down on a cardboard cake round. Have your frosting ready to go at room temperature or slightly below. (If your icing has been in the fridge and feels firm, you can beat it a bit to help smooth the consistency.)
Meet the offset icing spatula and cake rounds, your new best friends.
Using a bread knife, trim the edges of the cake to form a smooth, straight edge all around. Eat...ahem, discard...the excess.
Time to tackle the sides. If you're feeling risky (go for it!), lift the cake off the counter using the cardboard base. This will put the cake at eye-level so you don't have to hunch to see the sides. It'll also allow your hand to become a lazy Susan of sorts.
Alternative: Scoot the cake to the edge of the counter so one side hangs slightly over.
Like you did on the top of the cake, slap another dollop of icing (one that's big enough to cover the side top-to-bottom) onto the rim.
Using the back side of the spatula, push the icing around the sides, leaving about 1/4 of an inch all around. Grip on to the cake board to rotate the cake itself, not the spatula.
Add dollops as needed.
Be sure the icing reaches all the way down to the cake board and up to the surface of the cake. It's fine to leave some extra up at the top—you'll take care of that in a moment...
Hey—look what you did! Serve as is, or decorate as desired.