Prepare the watermelon by removing the seeds and cut into small cubes. Freeze in a large container for at least 12 hours.
Once you're ready to prepare the watermelon slushie, remove the watermelon cubes from the freezer and let them sit at room temperature at least ten minutes.
In the meantime, prepare the lime juice and mint leaves.
In a high-speed blender, add the ingredients and start to blend on low speed. Use the tamper to push down the pieces to encourage blending. Avoid over-blending though - if you're after that lovely icy consistency. This step involved frequent stirring to ensure all the cubes are blended and mixed with the mint.