An offset spatula or bench scraper is the best tool for making buttercream smooth. Using a turntable to spin the cake as you smooth the frosting is also key. Don't forget to put enough frosting on your cake. You want enough so that you can slowly scrape the outside layer of frosting off as you spin the cake, revealing the smooth frosting underneath! You don't need so many tools to make buttercream smooth, but you will need good technique!