Here's what you need to do:
1. Using your favorite lemonade mix, make some double strength lemonade. (So, if the directions call for 1/4 cup powder, make it 1/2 cup instead but use the same amount of water. Or if it's frozen concentrate, and it calls for mixing it with 3 cans of water, only use 1 1/2). Pour your strong lemonade into ice cube trays and freeze.
You'll also need some watermelon, so cut up your watermelon into chunks. (This is a perfect way to use up some of those end slices or the extra watermelon that you have when you use a melon baller). You'll want your watermelon chilled, so either keep it refrigerated, or pop it into the freezer for a few minutes if it's room temperature.
2. Once your lemonade is frozen and your watermelon cut up and chilled, put your watermelon in the blender and blend until liquified. You'll find that once blended, you have about half as much. I typically blend up about 4 cups of watermelon at a time and end up with a little more than 2 cups worth of watermelon lemonade.
3. Now that you have your watermelon all blended, add some lemonade ice cubes and blend some more. The amount you add is up to you. The more ice cubes you use, the thicker your frozen drink will be. I usually add 6-8 and it's drinkable with a straw, 10+ and I can eat it with a spoon.
That's all there is to it! It's best to drink your frozen lemonade shortly after making it. The watermelon pulp gets blended in with the frozen lemonade, so it's all nicely mixed together. However, if you let it settle too long, the pulp will rise to the top and the liquid lemonade will settle to the bottom. Also, we like the how these have just a hint of the lemonade flavor. You could also make your lemonade even stronger before freezing if you decide you want more of a lemony taste.