Cut off the top of the watermelon. Scoop the flesh out with a spoon. Remove seeds if your watermelon is not seedless.
After, put the de-seeded flesh in a blender. Set aside the watermelon bowl. Blend the watermelon flesh for 1 minute, until smooth. Strain the flesh with a strainer and remove the foam from the top with a spoon.
Measure 4 cups of strained watermelon juice and put it back inside the watermelon bowl. Add 4 cups of soju and mix well.
Put the remaining watermelon juice in an ice cube tray. Freeze.
Serve your watermelon soju with watermelon ice cubes.