In a cocktail shaker, muddle the mint, watermelon juice, lemon juice, simple syrup, and gin.
Add ice and shake well.
Strain into short cocktail glasses with fresh ice.
Garnish with more fresh mint or dried edible flowers, if liked.
Notes
I made the watermelon juice by blending fresh watermelon in a Magic Bullet then pushing through a strainer to remove most of the solids. You can use store-bought watermelon juice if you prefer but it will be sweeter so you can skip adding the simple syrup (unless you need it based on taste preferences).