Use a large Chef’s knife to slice the watermelon in half through the middle (not lengthwise).
Flip the half over onto the flat base, then cut it into slices. To cut a watermelon into triangles, simply cut the large slices into wedges. To cut it into cubes, go to the next step.
Switch to using a small paring knife (using a smaller knife gives you more control). Slice off the rind of each slice.
Switch back to the Chef’s knife. Make a grid pattern to cut the watermelon into cubes.