Here's what you need :)
Cut out the top of the watermelon.
Carve it with an ice-cream spoon. (I guess you could use a normal spoon as well but it would be easier with an ice-cream one)
Put the content of the watermelon in a bowl and remove its seed.
Strain the watermelon and put it back in its rind.
Pour the vodka inside it :)
You can put it in the fridge for a couple of hours and use some ice to make it more refreshing.
Soon your super easy cocktail is ready! (You can refuel the rind a couple of times :D) Consume it with pleasure and a jolly crowd!