Pass the icing sugar through a fine sieve into a large bowl. Make a well in the centre and add egg whites. Mix in a little sugar from the side of the bowl; add lemon juice and glycerine. Beat until smooth and glossy.
To ice your Christmas cake with royal icing,
Wait for the marzipan to dry on the cake for at least 24 hours. Brush the marzipan with a bit of water, then spread the royal icing. Allow the icing to harden, then decorate as desired.