Carefully cut the whole watermelon into quarters which will be easier to manage. Cut each quarter into smaller pieces. Use a sharp knife to remove the rind and all of the white leaving just the red flesh of the watermelon.
With a mandolin or a sharp knife, cut the watermelon into pieces for dehydrating. You can make the pieces whatever size you desire, but keep in mind they will shrink up to 50% in the dehydrator. Place the cut up pieces in a colander to drain for a few minutes while you work.
Once the watermelon has drained a few minutes, place the pieces on the dehydrator trays. Place the tray once filled on a cookie sheet or in the sink until you have filled all of the dehydrator trays. This is so the juice does not pool in the bottom of the dehydrator as you work.
Once all of the trays are filled place the lid on the top tray and turn the dehydrator on. Dehydrate for 8 to 11 hours or until the pieces are evenly dried. The watermelon will be tacky and flexible once dried completely. Allow the dehydrated fruit to cool to room temperature before storing it in an airtight container.