I was just browsing ao3, and I noticed that there are a few fics that have an enormous summary. There was one summery that included the entire back story for one of the characters.
When you write a summary, try to keep it short. Any more than five or six sentences, and it looks like you’re insecure in your writing. Or like you want to include information that isn’t in the fic itself.
Never, ever include things in the summary if they’re not addressed in the story.
There are also a ton of people on tumblr that would gladly help with summarizing, if you’re struggling with it.