By getting videogame companies to pay for the ratings.
It's the same thing as the ISO9000 scam. A bunch of people invent a standard, they make it seem like it is a good thing to have. And then they impose it on companies, who willingly pay to receive an arbitrary classification. As soon as the standard becomes popular, they set it up better and then companies, who fail to see how the ISO standard was establish, start paying like gullible fools.
And us, the consumers, are not in anyway better. We see ISO9000 certification and we assume that it actually means something important.