Wash the oranges thoroughly to remove any wax or impurities. Slice the oranges into uniform ¼-inch thick rounds, removing seeds as you go.

In a large skillet, combine three cups of granulated sugar and three cups of water over medium-high heat. Stir until the sugar fully dissolves and bring to a gentle boil.

Add the orange slices to the boiling syrup, arrange in a single layer, and reduce heat to low. Let the oranges cook for 45–60 minutes.

Once translucent, remove the slices from the syrup and place in a single layer on a wire rack. Allow them to dry uncovered at room temperature for 24–48 hours.

Check the slices; they should be tacky but not wet. Store them in an airtight container, separated by parchment paper.