Step 1: Wash the watermelon. Cut the watermelon into large chunks, removing the rind and any large black seeds. Place the cubed watermelon into a high-speed blender. If your blender is small, you may need to do this in batches.
Step 2: Add the fresh lime juice and a few mint leaves (if using) to the blender. Blend on high speed until the watermelon is completely smooth and liquidy. No need to add water, as watermelon is mostly water already.
Step 3: For a perfectly smooth juice, pour the blended mixture through a fine-mesh sieve or a nut milk bag into a large pitcher or bowl. Use a spoon or spatula to press down on the solids to extract as much juice as possible. Discard the pulp.
Step 4: Taste the juice and adjust if necessary. If your watermelon wasn't very sweet, you can add a touch of honey or maple syrup, but it's usually not needed. A little extra lime juice can enhance the flavor further.
Step 5: Chill the watermelon juice in the refrigerator for at least 30 minutes to an hour, or until very cold. Serve over ice, garnished with a fresh lime slice or a sprig of mint. Enjoy your homemade watermelon juice recipe!