Dehydrate citrus slices using the instructions linked in the article. To make ornament making easier, make a cut on each slice from the outside to the center.
Lay out the dehydrated citrus slices and double check that the slice you made in each pice of citrus is still clear and open. Be gentle; since the slices are now dry, they're more easily broken.
Match two slices of roughly the same size. You'll nest these two slices together to make an ornament, so be sure you like how they look together.
Line up slices on each piece of citrus and tuck them into each other so they're connected. Gently make sure that they fit well together.
Using a long piece of twine, you're going to basically wrap the center of the ornament to provide stability and to hang it. Starting at the top, with one length of the twine on either side of the ornament, bring the string all the way down to the bottom of the ornament.
Cross the strings over at the bottom of the ornament and bring them back up to tie a simple square knot. Don't overthink this! You're simply wrapping the center of the ornament vertically in twine.
Leave a few inches to make a loop and tie another square knot and then clip the twine. You can use the loop you just made to hang the ornament on the tree, or you can add an ornament hook to the loop.
To add fresh or dried herbs to the ornament, simply bundle them in small batches and use the twine to tie them to the top of the ornament. Keep them small so as not to overwhelm ther citrus slices.