You can ice cakes using a spatula and put a "crumb coat" on the cake to seal in the crumbs prior to decorating. Start by putting a large dollop of icing on top of the cake, spreading it completely over the top with a spatula. Use enough icing that you don't pull the crumbs into the icing as you work.
To cover the sides of the cake, use a fatter angled spatula and slowly turn the cake around as you spread the icing. A turntable is very helpful here.
The consistency of the icing is extremely important. It must be thin enough so that it doesn't pull the crumbs off of the cake. To smooth out any lines that are left on the top, put the spatula in hot water and wipe it off, and the heat in the spatula will help smooth out the icing.
Now that you have a nice smooth crumb coat on your cake, it's all ready to decorate.