You should only use royal icing on a cake that has first been covered with marzipan and allowed to dry out for at least 48 hours. This will help prevent the oil from the marzipan seeping through the royal icing and causing unsightly discolouration. The marzipan needs to be as flat and neat as possible with crisp sharp edges.
Fruit cake is usually covered using three layers of royal icing. However it does take a long time to do. You need to allow with 24 hours drying time between doing the sides and top and a further 24 hours minimum between coats. It is best to allow a week to cover a royal iced cake properly.
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:
Spread a thin covering of royal icing over the top of the cake using a palette knife.Then remove any excess from around the edges. Holding the straight edge at a 45degree angle, pull it across the top of the cake towards you. Lift it off and again remove any excess from around the edges. Leave the cake to dry preferably for at least 24 hours.
Spread a covering of royal icing over one side and remove the excess from the top edge and cake board. Hold the cake scraper vertical and run it along the side of the cake. Once more clean up the top edge and cake board. Then repeat on the OPPOSITE side. Leave the two sides to dry for 24 hours then royal ice the two remaining sides. To achieve a perfect finish apply two or three coats allowing 24 hours drying time between coats.