Cream together the caster sugar, butter and lemon zest until pale and well whipped then stir in the flour and ground almonds. Bring the mixture together into a ball with your hands then wrap in clingfilm and refrigerate for 45 minutes.
Preheat the oven to 140°C (120° fan) | 275F | gas 1 and line two baking sheets with greaseproof paper.
Roll out the dough on a lightly floured surface to 5 mm thick. Use a Christmas tree-shaped pastry cutter to cut out as many biscuits as possible, rerolling the trimmings as necessary. Make a small hole in the top of each biscuit with a skewer if you would like to hang them up with string later.
Transfer the biscuits to the prepared trays and bake for 25 minutes or until firm and just starting to colour at the edges. Transfer the biscuits to a wire rack and leave to cool completely.
Sieve the icing sugar into a bowl and stir in the lemon juice a few drops at a time until you have a stiff, pipable icing. Transfer half of the icing to a piping bag fitted with a plain nozzle.
Spread a third of the biscuits with the rest of the icing, then use the piping bag to pipe outlines, tinsel and baubles on the rest. Decorate the biscuits with silver balls, cake sprinkles and shimmer sugar before the icing sets.
Leave the icing to set for 1 hour before threading with string and dusting with a little icing sugar.