I don't use a crusting buttercream, but the basic frosting principles are the same. One thing that helps me is to have a cake board that is just slightly larger than the cake. Usually, your cake with shrink a tiny bit while baking so that you have a small lip, about an 8th of an inch between the edge of the board and the edge of your cake. If you're cutting it yourself, to size, or not using a board, then I recommend trying the pre cut boards.
Anyway, get a bench scraper and turntable, as mentioned, and you should find it easier.
I look at icing a cake as taking away the frosting rather than adding more. So, a good crumb coat is essential. Slap that on there, smooth it, and get it in the fridge. This will keep your bare spots to a minimum when you do your final coat.
Then put your cake at the center of the turntable, use an offset spatula to pile on a good amount of BC to the top and then the sides. It will be messy, don't worry about that, just be sure that every part of your cake is covered all the way to the edge of that board. Then you take your bench scraper and place it at a 90 degree angle to the cake, with one edge on the turntable and the other on the edge of the cake board. Slowly turn the table and pull your scraper along the edge of the cake. I tilt my scraper towards me so that it's at a 45 degree angle to the cake. I find that it scrapes off the overlap and redeposits where there are holes or bare spots.
clean off your scraper and check your cake. You will have a lip at the top, don't worry about that, yet. Look for spots where you need to fill in an air bubble. use your offset to add a little BC to those spots. Then take your scraper and go around again. Keep doing this until your sides are smooth.
Then work on the top. Many use an offset spatula, I like the scraper. Use what works for you. I place my scraper on the opposite side of the cake and pull towards me, turn the cake and do it again. I look at it as knocking off that lip. Be gentle, pull and turn. Then use your offset to smooth the center. Keep taking off small edges until it's as smooth as you can make it.
If you are using the paper towel method, this will be where you smooth those final imperfections, but that method won't do anything to make your BC coat even and flat. You have to get it almost perfect first.