When you look at your cake you don't want to see any bump, lumps, and especiall no crumbs!
Getting a perfectly smooth and bump-free cake, while keeping the crumbs out of your icing can be a bit tricky - but not impossible. All you need is a few easy tricks and cake decorating tools to make it all happen.
If you're using a spatular to smooth the icing on your cake then it's pretty easy. You can either have your cake on the table or you can place it on a cake turntable.
I like to use a metal spatular for this because I find it works much nicer than anything else I have tried. The trick here is to glide your spatula over the icing, at the same time never allowing it to touch the cake surface, as this will pull crumbs off the cake and into your icing.
To crumb coat your cake apply a thin layer of icing to your cake. At this point you don't have to worry about crumbs, this will come at the final icing stage.
After you've lightly iced your entire cake I always like to put my cake in the fridge or freezer to let the icing stiffen up a bit, for about half an hour.
You don't have to worry about making your cake look beautiful at this point.
Once you have pulled it out of the fridge, smooth all crusty edges with a knife or spatula carefully.
Then take the rest of your icing and spread a second layer overtop of your entire cake.
This is also an important step you'll have to take if you want to cover your cake with fondant. This is beacause you want your fondant to be smooth and lump free. You also don't want to see any of the cake showing through, especially if it's a dark cake like chocolate.
You'll also hear this method being called "dirty icing" the cake before you apply anything. Use the same method as you would above, put the cake in the freezer for about 15 minutes and then apply one more smooth, final layer. Remember, if you've got a good, smooth base to your cake your fondant will sit very nicely!
Here are some really great tutorials to help you get through your crumb coating!