Line two baking trays with silicone baking mats. Preheat the oven to 75°C.
Purée the watermelon, then strain it through cheesecloth set over a mesh sieve.
Drain out most of the liquid and set it aside for another use.
Combine the watermelon pulp with sugar, removing any seeds. Blend again if needed to make sure the mixture is smooth.
Divide the watermelon mixture between the trays, spreading it out evenly to about 3mm thick. Bake for 3 hours or until it’s no longer wet, but still slightly tacky to the touch. Rotate the trays 180° and swap their positions in the oven each hour until done.
Transfer the fruit leather to a chopping board and cut into your preferred sizes. Store between sheets of baking paper in an airtight container.