1. Remove the rind from the watermelon, cube into small pieces, saving a few for garnish and puree the remainder.
2. Pour the watermelon puree through a fine mesh sieve into a bowl and discard the pulp.

To make the cocktail:

1. Pour 4 ounces watermelon juice, tequila and lime juice into a shaker and stir. Pour into a Collins or old fashioned glass filled with ice and top with club soda.
2. Garnish with a piece of watermelon and lime wedge.
3. Pour remaining watermelon juice into a pitcher and save for later use or to make additional cocktails.