I've seen cakes that have the fat vertical ruffles on the side that are done with a large icing tip, but for me that's WAY more icing than you need to have on one piece of cake. Doing the ruffles that go around the entire cake lets you avoid the unnecessary icing buildup.
It doesn't have to be perfect, but if you're not good at staying in a general circle shape, you could mark a spiral to use as a guide on the crumb-coated cake before you start to pipe the icing on.
I did the side ruffles first on this, one, but if you do them after the top you can hide the edge of the top with the top row of ruffles on the side.
To do the side ruffles, just go around the cake with the thin edge of the tip pointing up and the thick side against the cake. After the first row move down and go around again and again until you reach the bottom.
If you're not good at staying level either, you can mark off the rows on the cake as a guide before you start.
When you get to the bottom, just fill in the ruffle, or pipe a flat band around the base.
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA