Start by preparing the orange. Use a vegetable peeler to peel the rind off the orange in long strips. Then, use a large, sharp knife to cut the peelings into very thin ribbons.
Bring a small saucepan of water to the boil. Reduce the heat to a simmer, then add the orange peel strips and simmer for 3 minutes.
Drain the orange peels through a heatproof sieve and rinse thoroughly in cold water. Set the peels aside and wash out the saucepan.
Place the white caster sugar and the water into the saucepan. Set over medium heat and allow the sugar to dissolve (do not stir at this stage, or the syrup might crystallise).
Once all of the sugar has dissolved, bring the syrup to a steady boil, then add the drained orange peels to the pan. Turn the heat down to low and cook the peels in the syrup for 15 minutes.
After 15 minutes, pass the peels through the heatproof sieve to remove the syrup (you can reserve this orange syrup to use in drinks later!).
Transfer the drained peels to a bowl containing the granulated sugar and toss with two forks until all of the peels are coated.
Remove the peels from the sugar bowl (you can pick them out individually or use a dry sieve to shake away the excess sugar). The excess sugar can now be discarded (or, like the syrup, used for some orange-y drinks).
Spread the peels out on a baking tray, lined with greaseproof paper, making sure they are in a single layer. Set them aside in a cool, dry place, overnight to dry out, then transfer them to an airtight container and store in a cool, dark place until ready to use/serve. The peels can be stored like this for 6-8 weeks.