For a single cup I do about 1-2 cups of watermelon, 1/2 C limeade and a good handful of ice and I add more limeade or ice to get the consistency I want.
For a full pitcher I fill my blender about half full with watermelon and 1/3 full of limeade and then ice to make it good and slushy.
Just blend it until it’s pulverized. I like mine more drinkable but it’s really yummy as a slush too.