To fill a Christmas tree, start by hanging the lights. Make sure to use a ladder or a stool if you can't reach the top of the tree. Then, decorate the tree with ornaments, garlands, and other festive items. Make sure to spread the decorations evenly around the tree, so it looks balanced and full. Finally, add any finishing touches, such as a tree topper or a wreath, and stand back to admire your handiwork!