Start with navel oranges. I bought about five and got about 20 ornaments out of them.
Preheat oven to 225°F.
Next blocked your orange slices on the kitchen towel or paper towel to release the excess moisture.
Place the orange slices on a baking rack and place in the oven on the middle rack.
Here is how mine look like after they came out of the oven:
I used mine for both my advent wreath and for Christmas tree decorations.
I poked a hole in it with a large toothpick to make a threading whole (is that a word?) lol... For my jute to string.