Freeze the half cup of watermelon a day before you want to make this drink. 

Puree the two cups of watermelon cubes in a blender. Use a strainer to strain out any seeds and pulp. Add the strained watermelon juice back to the blender. Pour in the green tea and run the blender on low to mix gently. 

Add the mixture to a large glass container. Garnish with mint leaves and the frozen watermelon cubes.