To crumb coat the cake, I put a layer of icing on top of one cake and topped it with the other cake. Make sure to choose the smoothest and flattest of the cake surfaces for the cake top (it is very likely going to be the bottom of one of the cakes), this will make it easier to ice and decorate the cake. Then I crumb coated it with icing. Start from the top, then do the sides. Use a cake spatula to make sure the surface of the cake is as smooth as possible. Then put it in the fridge to harden. This coating will make the fondant top look smooth and it will ensure that no crumbs will be visible through it.