It looks a little tricky to get those first cuts into the sides of the melon, but it keeps it contained nicely.
My sister is a chef. This is how she showed me to do it:
First cut the melon in half. Lay it flat side down. Cut off the rind starting at the top and down. Get out the white off. Then you can cut it into slices. If you want chunks then lay a couple of slices on top of each other and cut one way and then the opposite.