Extract the Juice: Cut watermelon into chunks and remove any seeds. Place the chunks into a blender and blend until smooth. Strain through a fine mesh sieve to get fresh juice.

Mix Ingredients: In a shaker, combine the gin, watermelon juice, simple syrup (if using), and lime juice with ice. Shake well.

Prepare the Glass: Fill a glass with ice cubes.

Pour and Top Up: Strain the mixture into the prepared glass. Top up with club soda or sparkling water.

Garnish: Garnish with a slice of watermelon and a sprig of mint.

Serve and Enjoy: Give it a gentle stir before serving.