This achievement can be a little confusing at first, as when you attempt to do this, it may seem like there is no way to do this. It turns out that you can only rate the difficulty of levels that haven't been rated yet. To do this, search for levels with a "NA" difficulty. There should be a star icon on the right, below the normal like/dislike button. You can use that to rate how many stars you thing the level should be worth, granting this achievement.
Please play the level before you give it a rating. While I don't think it is strictly necessary, try to be respectful to the level creator and the community and give an honest rating.
If you have any ideas on how I can improve this solution, I'd love to hear them! There may be something that isn't here that should be, or an easier way of doing this. I'm always open to suggestions. If you notice any typos or poorly phrased sentences, please let me know.