Place the diced or cubed fresh watermelon in a saucepan.
Add granulated sugar to the saucepan, starting with ½ cup, and adjust according to your desired level of sweetness.
Heat the mixture over medium heat, stirring constantly to dissolve the sugar.
Bring the mixture to a simmer and allow to cook for about 10 minutes, or until the watermelon pieces have softened.
Use a potato masher or a spatula to press the watermelon and release all the juices.
Remove the saucepan from the heat and let the watermelon syrup cool.
Once cooled, strain the syrup through a fine-mesh sieve or cheesecloth to remove any remaining watermelon solids, leaving you with a smooth liquid.
Transfer the watermelon simple syrup to a clean glass jar or bottle for storage.
Seal the jar or bottle tightly and refrigerate the syrup until ready to use.