What you need:
Nail Polish Remover
First, you'll have to put nail polish remover on your cotton ball. Make sure it's pretty wet, but no dripping.
Place the cotton ball on top of your nail.
And wrap the square of tin foil around it.
Wrap all your fingers like this, and let them sit for about 5 minutes.
Then, slide off the foil. If there are still a few polish streaks, you can rub it off with the cotton ball.
Ta-da! It works very well, be sure to try it out!