Blend about 7 cups cubed watermelon into juice, then strain to make 4 cups of watermelon juice.
Squeeze 5-8 lemons and strain pulp and seeds out to make 1 cup fresh lemon juice. Discard the pulp.
In a 1 gallon pitcher, combine water, white granulated sugar, lemon juice, and vanilla extract. Mix well until sugar is completely dissolved.
Add the strained watermelon juice last and mix it in well.
Crush up mint and put it in the pitcher right before serving, or add a few leaves to each glass for a slightly minty taste. I like to do it in idividual glasses since not everyone likes the mint. Add ice to serve, garnish optionally with mint sprigs, lemon wedges, or small sliced watermelon wedges.
Notes
This recipe used 7-8 cups of cubed watermelon, which was almost ¼ of a watermelon for me. You can throw away the pulp, compost it, or I like to freeze it and eat it! Store refrigerated for up to 3-5 days, or freeze in an airtight container for up to 3 months. It's normal for the watermelon juice to separate a little when refrigerated, just give it a little stir and it will be fine!