I need to frost a cake with royal icing. Where do I begin. Royal Icing is hard to work with. How do I get the cake smooth?

You first need to cover the cake with either marzipan or fondant. RI will not smooth properly if you ice it straight onto cake.

It takes a few days to ice a cake with RI. You have to thin it out some from decorating consistency to make it more easy to spread, and then let it dry for about 12-24 hours between coats. It takes about four coats of RI to get it smooth and sturdy enough to handle decorations without cracking.

You might want to check for some Austrailian or British fruitcake wedding cake tutorials. The RI technique is more common there.

HTH!

CeeTee is absolutely right. This is a link to a step by step
http://www.bsguk.org/forum/viewtopic.php?t=6195

HTH