Be sure to push the spatula through the icing. To avoid cake crumbs*, always push the icing from the middle outwards. Then lift the spatula and begin again in the middle, going out in each direction. When it's time to evenly spread the frosting, don't backtrack. Keep the spatula on the frosting. Don't let it touch the unfrosted cake unless there's plenty of icing on the spatula.
*I rarely have problems with crumbs when icing a cake, because I let my cake sit for a couple of hours before I frost it. After a while, the crumbs are absorbed back into the cake.
Basic Cake MakingHow to Make Buttercream IcingCake RecipesFrosting and Icing Recipes