First install web extension EditThisCookie from http://www.editthiscookie.com/ Then open http://r00tz-web-intro.appspot.com/rootz And solve all challenges in it. They are all damn easy. when you have solved all levels in it and paste it's cookies in http://xss-game.appspot.com/ and refresh the home page. And you will see that you have completed all the levels.
A simple tech blog mainly consisting of security which is most important in today's world