I would use Royal Icing to join the pieces together, and then use a softer buttercream type icing on the outside to make it look pretty. Royal icing sets like a rock, so use it sparingly.
For royal icing, i use half an egg white (beat it with a fork) and then add pure icing sugar and 1tsp of fresh lemon juice (optional). You need to mix it until it has a shiny appearance. I mix small amounts in a coffee mug, and use a butter knife to mix in the pure icing sugar. It usually takes around 5mins for it to become nice and glossy. Dont use it until you have the glossy appearance.
If you need to store it to use a little later, then place a damp piece of paper towel over the cup, and cover with cling wrap. Make sure the paper towel stays moist.
Best of luck and Merry Christmas.
