Like the achievement says, you simply need to like an online level. To do this, after playing through a level, simply click on the thumbs up/thumbs down symbol on the right, and it will bring up a page where you and like or dislike a level. For this achievement, you have to specifically like a level.
Please play a level before you like it. Its disrespectful to the community to like a level you haven't even touched.
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.