First, you will want to cut your watermelon in half, length-wise.
Then, cut each half into half again, again length-wise.
Working with 1/4 of the watermelon at a time, line the length of the watermelon up to face you. Then, using a long knife, cut slices straight down into the watermelon without cutting all the way through the rind.
You will then turn the watermelon and make 2-3 slices parallel with the slant of the watermelon.
Turn the watermelon and repeat step 4 on the other side.
Lastly, run your knife around the edge of the watermelon, where the watermelon turns white.
Dump the cubed watermelon into your desired bowl.
Repeat with the remaining 3 quarters of watermelon.