The above process is great for stubborn stains, but can be abrasive on the enamel. Instead, try diluting a teaspoon of vinegar with a cub of water and use as a mouth wash.
The trick with using apple cider vinegar as a teeth whitener is consistency. You won't see results after just one go. You'll need to consistently use it everyday for at least a month. Like all good things, it takes time!