Heat the oven to 170°C/340°f/ gas 3 (150°C for fan ovens), and cut a piece of cardboard into a tree shape about 20cm high.
Make the gingerbread mix according to packet instructions.
Roll out the dough to a thickness of 4mm, then use the cardboard template to cut out two tree shapes per single gingerbread tree you are making, cutting one in half vertically.
Transfer to a lined baking sheet and bake for 10-15 minutes, then cool.
When ready to assemble, make royal icing by whisking the egg whites until they foam.
Beat in the icing sugar until thick and smooth.
Decorate each biscuit with icing, then join together using icing, holding in place until it sets.