I don't know if most of you already know how to do these but I think it's less common in Spain .
The first thing to do preheat the oven at around 150º.
Cut your orange slices about 3-4 mm I'm guessing .
place a wire rack inside a baking sheet and lay the oranges with enough space between them so the air is going round nicely.
Keep an eye on them, if necessary turn down the temperature.
it's going to take between 6 - 12 hours so make sure you do as many as possible in one go.
when ready and cooled down keep them in a sealed plastic bag/container.