Coating a fruitcake with royal icing will allow to you to achieve sharp, crisp corners and edges. Royal icing is only used on fruitcake which should have been covered with marzipan first. The top and sides should be as neat and flat as possible as any lumps and bumps will show through the icing. The marzipanned cake should then be allowed to dry out for at least 48 hours to prevent the oils from the marzipan seeping through the royal icing causing discolouration.
Royal icing is not suitable for covering a sponge cake although it can be used for piped decorations on a sugarpasted sponge.
Fruit cake is usually covered using three layers of royal icing with 24 hours (minimum !) drying time between coats. You also need to leave 24 hours drying time after you have done the sides before Royal Icing the top so you do need a lot of patience !
The royal icing that you use should be of the soft peak consistency (soft and creamy and easily spreadable) so place a couple of spoonfuls into a bowl and beat in a few drops of water if necessary. Cover the bowl with a damp cloth when not in use to stop any remaining royal icing in the bowl from crusting over.
You will need a turntable to get the best results.
You will need:
Using a palette knife, spread a coating of royal icing around the sides of the cake.
Hold the spreader vertical and stationary, against the side of the cake. Grasp the turntable as close to the hand holding the scraper as you can and slowly rotate the turntable in one complete rotation. As the cake rotates, excess icing should collect on the spreader leaving the side of the cake smooth and covered with a thin coat of icing. Lift the spreader away from the cake.
Using a palette knife carefully remove the excess royal icing from the top of the cake. Allow to dry for at least 24 hours before covering the top.
Spread a liberal coating of royal icing over the top of the cake. Holding the straight edge at a 45 degree angle, pull it across the cake towards you then lift it off. Neaten the edges using a palette knife. Leave the top to dry for at least 24 hours then repeat on the sides and top another two or three times allowing 24 hours drying between each section. (I told you you needed an awful lot of patience!)