First you need the nail polish. I used a normal nail varnish for the red base and green, white and black nail art pens. You can easily use normal nail polish for these colours and a toothpick for the black seeds.
Paint your nail completely red and wait for it to dry.
Paint the tip of your nail with the green and wait for it to dry. It doesn't matter if the edges are a bit messy as it will be covered up. :D
Paint a line of white between the green and red. Mine was a bit slanted this time, but it looked ok when it was finished.
Finally dot some black onto the red to make it look like the watermelon seeds.
Finish with a topcoat and there you have some pretty watermelon nails. Happy painting! :D