Place your watermelon on the cutting board. Carefully cut it into large sections. Cut each section away from the watermelon rind. Dispose of the rind and cut the watermelon flesh into 2 -3 inch pieces. (These are getting blended up, so don’t worry about them being symmetrical or looking nice!)
Place 3-4 cups of diced watermelon in a blender, cover, and purée until smooth.
Pour the pureed watermelon through the fine mesh sieve into the large bowl. Use a spoon or spatula to move the watermelon pulp around in the sieve, making sure all the juice goes into the bowl.
Dispose of the pulp leftover in the sieve.
Repeat steps 2-4 until you have all the watermelon juice you want!
Pour the watermelon juice from the bowl into a pitcher and chill until ready to use.