Need some tips on frosting a cake ?

I baked a cake earlier today with my son. We let it cool for about an hour. The problem came when it was time to add some frosting. I have a special spatula made for frosting a cake but when we got started crumbs from the cake mixed with the frosting. It is a chocolate cake with white frosting so you can imagine the cake crumb pieces stand out. If this helps the cake was a box cake and the frosting came from a can. The brand of frosting was Betty Crocker whipped frosting. Can anyone give me tips on how to keep cake crumbs from mixing with the frosting? Serious answers only.