http://www.thekitchn.com/how-to-cut-up-a-watermelon-step-by-step-tutorial-221100

You need:
1 whole watermelon
Cutting board
Large, sharp knife
Large bowl or plate

Place on a cutting board: Start with a whole, half, or quarter watermelon, and place it on a large cutting board.
Cut the watermelon in half: If you're using a whole watermelon, cut it in half lengthwise.
Quarter the watermelon: Cut each half in half again, so the whole watermelon is now quartered.
Slice into wedges: Working with one quarter at a time, make slices 1 to 2 inches wide, across the melon, cutting just down to the white rind. Do not slice all the way through the bottom of the melon.
Make lengthwise slices along the melon: Starting on the side of the watermelon about an inch below the peak, insert your knife with the tip pointing down. Make a lengthwise slice across the quarter, cutting down so the tip of the knife runs along the rind. Do not cut through the bottom of the watermelon.
Repeat: Make another lengthwise slice 1 to 2 inches below the slice you just made. Again, slice along the length of the quartered watermelon, with the tip of the knife angled down so it runs along the rind, without cutting through the bottom of the melon. For larger melons, you may need to repeat this step a third time.
Turn and repeat: Turn the watermelon around and repeat on the other side.
Slice to remove the flesh from the rind: Use a sweeping motion to slice between the watermelon flesh and rind. Hold the top pieces in place, if necessary.
Turn the watermelon into a bowl: Turn and pour the watermelon into a large bowl or plate.