Dice watermelon into 1/2-1 inch cubes and transfer to a sheet pan, spreading out the cubes in an even layer. To prevent sticking, we suggest lining the pan with parchment paper. Freeze for 2-3 hours (until frozen solid), then transfer to a freezer bag/container until ready to use.  It's important to freeze the watermelon on a sheet pan first (versus just adding it to a bag and freezing), otherwise the frozen watermelon will stick together in large clumps.