Puree the watermelon first to create the liquid base.
200 g fresh watermelon
Add the spinach and avocado.
2 cups 85g raw spinach, 1 oz 28g avocado
Mine barely fit. I had to squish it in.
Once you're ready to blend, if you are using an upside-down processor, make sure to give the cup a few shakes so the liquid is near the blades. Blend until smooth.