Simply spread a very thin layer of frosting over the entire cake. If you are layering, layer like normal and crumb-coat the whole layered cake. It does not matter if you lift up crumbs, just try to get the cake somewhat smooth. Refrigerate the cake until the frosting hardens and becomes smooth. The cake should frost quickly and easily with a perfect smooth finish.
Every person has their favorite method and utensils for frosting cakes.
Some people like rubber spatulas, other metal spatulas, and still others like plain butter knives. In my experience metal spatulas work best. Not the kind of spatula you flip hamburgers with, but a straight edge metal spatula with a sturdy handle. The spatula I use is hand made and whisper thin. It frosts like a dream.