SuggestionsYou can do a couple of things to help yourself. Once your cake has cooled & you've cut your layers, clean your work area to try & keep the area free from crumbs.
You want to spread a little bit of buttercream or "icing" at a time. Don't slather on a ton of icing at one time because it will "pull" the cake. Start with a small amount to give your cake a thin "crumb coat". Refrigerate it for about an hour or so. Bring the cake out & when you start your "final icing", make sure to swipe your spatula clean (each time) into another container or onto a towel to not put crumbs that may have gotten on there, back into your main bowl of icing. Does that make sense? So, spread from the main bowl of icing, decorate cake, wipe spatula clean, then back into the main bowl of icing again. This should help control your crumbs.
Also, don't get too stressed out over it. If you plan to decorate further, you can always cover your "mistakes" ;-). Good luck. You can email me if you'd like.