Place cubed watermelon in a blender and process until completely liquified.
Place a fine mesh sieve over a bowl and pour the watermelon juice through it. Let juice strain through and leave pulp behind. You should have about 1 cup of juice.
Pour juice into a mason jar and add sugar. If your watermelon is sweet, you'll only need ½ cup sugar. If it's not a sweet watermelon, use more. Close jar tightly and shake vigorously for about 2 minutes. Let rest and then repeat for another 2 minutes.
Put watermelon syrup in fridge to chill for at least 20 minutes before using. You'll need to shake jar again before using as ingredients will settle.