Summarize:

While your popcorn garland might seem like the finishing touch to your Christmas tree, it’s actually easier to hang it when there aren’t ornaments in the way. You should add your lights before the garland, though. The best way to hang popcorn strings on your tree is to softly drape them over the branches rather than firmly stuffing them in gaps. Start at the top and carefully work your way down.  For a formal look, make sure to drape your popcorn strings in even, uniform loops. For a more casual look, allow the popcorn strings to drape unevenly. If you want your popcorn garland to stand out on your tree, consider doubling up the strings and draping them together over the branches. Once the popcorn strings are in place, you can hang your ornaments on the tree. However, you should take care when placing them among the garlands because you don’t want any large, heavy ornaments to rest on the popcorn and possibly break it.
Add the garland after the tree's lights. Place the garland on the tree. Add ornaments.