Cut the watermelon into small cubes, discard the external layer and remove as many seeds as you can (it's ok if some seeds are left).
Place the chopped watermelon into a dish/tray/tub and place in a freezer until completely frozen.
Using a peel, peel a couple of rinds of the orange zest. Then cut the orange in half and squeeze out the juice. Measure out 1/3 cup of the orange juice.
Add the frozen watermelon, 1/3 cup sugar, the orange peel and 1/3 cup of orange juice into a blender or food processor. Then blitz until smooth.
Pour the blended mixture into a tub/tray/dish and place it into a freezer for at least two hours. In between this 2 hours period, take the tray out of the freezer and scrape/break the watermelon mixture apart every 30-40 minutes. By the 3rd or 4th time you should have a crystalized shaved-ice texture. Serve and enjoy!