To participate, you just need to do something kind, unexpected, and unnecessary for someone else between now and December 19th.
It can be as simple as letting the person behind you in line go ahead of you, grabbing the door for the woman struggling with the stroller, donating blood, or taking a plate of cookies to a neighbor. If you just keep your eyes open for opportunities, plenty will present themselves.
You can enter more than once, so take every opportunity to do that little something extra for someone else. Who knows, it could become a wonderful habit.