This is probably the most popular (and easiest) way to decorate a fruit cake with royal icing. It is quick and needs no particular skill whatsoever.
The fruitcake needs to be covered with a layer of marzipan and in position on its plate or cakeboard. Using a palette knife, spread a thick coating of royal icing over the top and sides of the cake. Clean the palette knife then use the flat side of it to swirl and tease the royal icing into soft peaks. Tutorial Swirly Royal Icing