The traditional way to put ribbon on a Christmas tree is to drape it from the top to the bottom in a zig-zag pattern, creating a cascading effect. This is usually done after the tree has been decorated with lights and ornaments. To start, begin at the top of the tree and drape a piece of ribbon across, going in a diagonal direction. Then, bring the ribbon back up and drape it across in the opposite direction. Repeat this process until the entire tree is covered. For a more modern look, you can also use ribbon to create loops or bows on the tree.