Use a clean, dry dish towel or paper towel; coarse fabric creates more texture. Give the cake a smooth coat of frosting and let stand until the frosting is dry to the touch, about 30 minutes. Fold the towel and, working with a flat side, press the towel lightly into the frosting and pull it straight off without smearing the frosting. Use light pats of the towel to smooth out any bumps or ridges. If the towel starts to stick, move on to a new area of frosting and let the sticky part dry a bit before toweling.