There are many different techniques out there from the paper towel method to a knife in hot water. Why do you want to add paper or water to your icing? The trick is to start off with a nice smooth soft buttercream in the container. Once you have this, smoothing out a cake will just take practice. Now I’m not knocking the people who use paper towels to smooth buttercream. The fact is the buttercream I use does not crust. So if I put a paper towel on top of my cake and tried to smooth it, I would either pull off all the icing or I would have to pull paper towel out of my cake. Now if I dipped a knife into hot water each time I had to ice a cake I would have water everywhere including all over my cake. Now that’s not to say I don’t use hot water I do, but just to heat up my spatula to smooth out small flaws in my cake. For example if I wrote on a cake and the name is spelt wrong, instead of scraping the whole cake I would just take off the name re-ice the area and smooth it with my hot spatula. Everyone can give tips on how to get perfectly smooth  buttercream. If you want to learn how to ice a cake smooth you just need the right equipment and practice.