Note: to make the fresh watermelon juice, I add about 2 cups of cubed watermelon to a blender and puree until it's smooth. Pour it over a mesh strainer into a bowl and you have watermelon juice! :)
Fill the glass with crushed ice. Add the grapefruit syrup and tequila and stir. Pour in the watermelon juice. Pour the soda over top and garnish with a watermelon or grapefruit wedge. Serve!
Heat the juice and sugar in a saucepan over medium heat. Whisk until the sugar dissolves and bring the mixture to a boil. Turn off the heat and let it cool to room temperature. It will become slightly syrupy. This stays great in the fridge for a few days.