Frosting a layered cake may seem like a daunting task, but this quick tip makes everything much easier: make a crumb coat.
A crumb coat is a thin layer of frosting that fills up all the nooks and crannies of your layered cake, and catches all the lose crumbs that could eventually end up on your final layer. To make one, simply use an offset spatula to spread a very thin and even layer in and around your cake. Let the crumb coat set in the refrigerator for 30 minutes (or 15 in the freezer), then continue with the final layer of frosting.
Watch how to frost a cake, crumb coat included, here: