The best way to hang Christmas lights on an outdoor tree is to first choose lights that are suitable for outdoor use and designed to withstand weather conditions. Once you have selected the lights, you should begin by attaching the lights to the top of the tree using a ladder and a staple gun or clips. Then, work your way down the tree, securing the lights at regular intervals. Be sure to take into account the height of the tree and the amount of lights you need to hang. Finally, make sure to test the lights before hanging them on the tree to ensure they are working properly.