To hang Christmas lights outside your tree, you'll need to have an extension cord, a set of lights, and some hooks or clips to attach the lights to the tree. Start by wrapping the lights around the tree in a circular motion, making sure to leave enough slack to connect the extension cord. Then, take the clips and attach them to the lights, clipping them to the branches of the tree. Finally, plug in the extension cord and enjoy your festive and illuminated tree!