Preheat oven to 200 degrees F. Slice oranges about 1/4 inch thick, discard ends. Choose the twelve largest, most uniform slices and place on baking sheet lined with parchment paper.
Bake for two hours. Flip oranges, then continue baking two more hours. Flip again and bake 30-45 more minutes, or until completely dry. Don't brown. Remove from oven and cool completely.
Assemble Sprigs
Cut wire into twelve 6-inch pieces. Poke 1/2 inch of a wire piece through an orange slice near the top. Bend long side of wire around back and twist with short side of wire three times.
Bunch three rosemary twigs together. Place wired orange slice near top, where the length of rosemary underneath is visually pleasing. Use your fingers to strip needles off rosemary above orange slice. Tightly wrap long end of wire three times around rosemary bunch. Loop wire under orange slice to secure. Snip bare rosemary twigs a little above wire wrapping.
Set sprig aside and repeat process for other 11 orange slices.
Assemble Garland
You'll be attaching an orange-rosemary bunch every 9 inches, starting 19 inches from each end. (This leaves a tail to curl if ribbon is wired. Feel free to adjust measurements to your preference.)
Measure and mark your ribbon for even placement of sprigs. At each mark poke wire through bottom edge of ribbon, loop it around back and then twist it around itself once. Twist twice around rosemary twig ends (where wire is wrapped from making the sprig) and tuck wire tail under orange slice. Repeat for every sprig.
Hang over a mantel, along a bookshelf, or anywhere a bit of natural Christmas cheer is needed!