The best way to decorate outdoor trees for Christmas is to start with a strong base of lights. Wrap the tree from top to bottom with strings of lights, starting at the top and working your way down. Next, add some ornaments, such as shiny balls or colorful baubles. Finally, finish off the tree with a topper, like a star or an angel. You can also add some extra touches, like tinsel or garland, to make it extra festive!