Using a sharp chef's knife, carefully slice the watermelon in half crosswise (not lengthwise) so you have two pieces that have a round, circular cut side (if you cut it lengthwise, you'd have an oval).
Starting with the first half: trim off the stem end of the watermelon so that you can see some of the red flesh easily peaking through the circle.
Turn the melon face down on the cutting board (so that the larger, fully cut side is flat against the board.
Starting at the top, stem end, hold your knife at an angle and gently cut down towards the cutting board so that you remove a strip of the rind. Cut deep enough to where the rind is fully removed and you see the dark pink flesh of the watermelon (remove all of the white/very light pink part. You'll lose some of the red with it, but that's OK.).
Rotate the melon and repeat, continuing until you have cut away all of the rind.
Cut the melon horizontally into thirds, so that you end up with 3 pieces that are each a full circle.
Cut each circle into cubes of your desired size.
Repeat with the second melon half.
Place in a container and chill in the refrigerator for at least 4 hours (for icy cold watermelon) or refrigerate for up to 3 days or freeze for 3 months or longer.
TO STORE WATERMELON CUBES: Refrigerate watermelon cubes in an airtight storage container for up to 4 days.
TO FREEZE: Lay the watermelon pieces in a single layer on a parchment-lined baking sheet, and freeze until solid.
Once frozen, transfer the pieces to an airtight freezer-safe storage container or ziptop bag and freeze for up to 3 months.