Dissolve 1/2 cup of sugar in 1 cup of warm water in a pitcher, and stick in the fridge.
Coarsely chop watermelon until you get 4 cups.
Place watermelon in a blender and blend until it is liquid.
Add in lemon juice, and blend again.
Place watermelon lemon mixture in the pitcher with sugar water and add 2 more cups of cold water.
Mix, place back in the fridge until you are ready to drink it and mix before drinking, or just throw in some ice cubes and enjoy! Yummm!
Notes
If you don't like pulp, feel free to use a strainer to strain it out before adding the lemon juice.
If you are in a hurry to make this, you could dissolve the sugar in less water and then add a few ice cubs. Or for a slightly faster way, you can pop it in the freezer for 15 minutes or so.
For the best flavor, use freshly squeezed lemon juice.