As you move around the tree, try to let the ribbon do its thing. The goal is for the ribbon to drape naturally and not appear forced. The ribbon should never look pulled or stretched.
You can also create curls in the ribbon by twisting it in your hands. This look is a bit harder to achieve but does add an interesting effect. I usually do this only 3 or 4 times per tree.
Continue this process as you work around and around the tree. Avoid working in a perfectly straight line. As you can see, my lines vary. Some are quite wavy; some are less so. You want to avoid sharp bands of ribbon that cut across the tree.