The rules are very, very simple: add an ornament to the Christmas tree. It doesn't have to be your average, everyday Christmas ornament. You can hang a Predator mask on there, or a dead chestburster, or Drake's bone necklace, I don't care.
The catch is that the ornament you post has to relate in some way to the post above you.
Right. We've got a big, nice-smelling balsam fir set up, and infinite boxes of stuff to put on it. There are already lights on it and a glowing figurine of the Almighty and Distinguished Evil Dictator Corporal Hicks sits on top of it. The first thing hung on the tree is . . . an Alien egg.