The traditional method of decorating a Christmas cake is with almond paste or marzipan and icing. You will need about 2 lb of marzipan to cover the cake.
Place the cake on the cake plate on which it will be presented.
Roll the marzipan out as thinly as possible and cover the top and sides of the cake. A thin layer of strawberry jam spread over the cake will help to 'glue' the marzipan to the cake.
To make the Royal Icing sift 12 oz. of icing sugar (confectioner's sugar for Americans) into a bowl. Beat two egg whites then fold them into the sugar. Add 3 tablespoons of lemon juice. Brush the marzipan with a little egg white to help the icing stick to the cake then add the icing in a thin layer. The icing can be thickened if necessary with additional icing sugar.
A flat metal spatulla dipped in hot water will help to spread the icing but in general best results are obtained if the icing is 'worked' as little as possible.