I haven’t added one myself, but this is my understanding: all Twitter users can opt into the community notes feature but at first you can only rate notes as helpful or unhelpful. Once you’ve rated 5 notes you have the ability to write your own. You can do so on any tweet, but if your note(s) don’t get enough “helpful” ratings, you lose your ability to write them. You have to go back to rating notes - once you’ve rated 5 more you can write again.

When you add a note you chose a reason why you’re adding it, similar to explaining why you’re reporting a comment on lemmy. If you say that the original tweet is misleading you have to say why and provide a source. It only shows on the tweet to everyone if you get enough “helpful” ratings from the people who have opted into the scheme.

Not very EILI5 but you get the jist!

https://help.twitter.com/en/using-twitter/community-notes