Cut the rind off the watermelon and remove as many seeds as you can.
Cut the watermelon into slices. Remove seeds while you are cutting the watermelon up.
Place the watermelon onto a baking sheet and put it into the freezer until the watermelon freezes. This helps to harden the watermelon before placing into the containers.
Remove the watermelon from the baking sheet once it is frozen and put into a resealable container. You can use an airtight plastic container or a resealable plastic bag. Place the now contained watermelon in the freezer.
Thaw the watermelon in the refrigerator and eat within 4 days of thawing.