For the Mocktail:

1. Rim the glass.  Pour some Tajin into a shallow dish. Rub a lime wedge around the rim of a glass. Dip the rim into the Tajín to coat. Set aside.
2. In a cocktail shaker or mixing glass, muddle the jalapeño slices with the lime juice.
3. Pour the watermelon juice and agave syrup into the shaker. Stir thoroughly to combine.
4. Fill the Tajin-rimmed glass with ice. Pour in the watermelon mixture.
5. Top with sparkling water and garnish as desired.

For the Watermelon Juice:

1. Add the watermelon cubes to a blender. Blend on high until completely smooth, about 30 seconds.
2. Pour the blended watermelon through a fine mesh sieve or cheesecloth into a bowl or pitcher to remove the pulp. Use the back of a spoon to press out as much juice as possible.
3. Store in the refrigerator for up to 3 days in a sealed container.