The best way to put Christmas lights on an outdoor tree is to use a ladder to reach the top branches. Make sure to start at the top and work your way down, so that you don't miss any areas. Be sure to evenly space the lights around the tree and use a combination of both clear and colored bulbs to create a festive and colorful look. Additionally, try to use lights that are rated for outdoor use and are safe to use in the elements. When you're finished, stand back and admire your work!