Peel the watermelon, if possible until the red pulp, cut it, put it in a large bowl and use an electric. Mash the hand blender, the remaining firm shell can now be easily fished out.
For me it was 2 liters, of which 600 ml were used for a cake, the rest was poured into a large saucepan.
Possibly follow up now.
Take out half a cup of juice to dissolve almost 120 g of cornstarch, this was then stirred into the boiling melon juice / pulp and gently boiled, CAUTION, it bubbles very strongly.
Pour into three deep plates rinsed with cold water, eat either hot or after cooling.