Cover the cake and set aside. Knead the 500g icing for the stars and roll out to 5mm thick on a surface dusted with icing sugar. Dust with edible lustre, then rub it in with your fingertips. Stamp 20 stars out of the icing using one 6cm cutter, or use two different-size cutters if you have them. Leave to dry overnight.
If you’re putting a ribbon around the cake, do this now. Start building up the hardened stars - start with three in a triangle shape, pushing the points into the icing on the cake for stability. Build on this basic frame, letting the stars support each other. Lay a few stars around the base of the stack. Cake can be iced up to a week ahead.
TO COVER THE CAKE: To cover a cake with marzipan, first brush the cake all over with a thin layer of warmed apricot jam. Dust the work surface with icing sugar, then roll out the marzipan evenly until you have a 5mm-1cm thick round, about 40cm across for a 20cm cake. Lift over the cake, using a rolling pin to help, then smooth with your hands and trim off the excess. Leave to dry overnight or for a few hours. Lightly brush the marzipan all over with cooled, boiled water. Roll the icing out as you did the marzipan, then smooth with your hands, trim off the excess and leave to dry.