Write down the essential elements of the story: when and where the story takes place, the main characters, what problems the characters face and how those problems are resolved.
Determine which characters are essential to the story. If your story has one main hero and one main enemy, focus your summary only on these two characters.
Decide which parts of the plot to include in your summary. Describe what the main character is trying to accomplish, the primary struggle he faces and how that struggle is eventually resolved.
Summarize the book in a single sentence to determine if you've selected the right characters and events from the story to include in your summary. If you were summarizing "The Odyssey," for example, you might write, "Odysseus is a Greek king whose return home from the Trojan War is delayed for 10 years by monsters, nymphs and the wrath of the gods."
Write a rough draft. Don't worry about producing the perfect summary yet--just get the basic outline of the book down on paper.
Read your rough draft and ask yourself if it includes all the essential elements of the story: setting, main characters and plot.
Write a second draft, making any necessary changes. Check that your summary is the right length and has a logical beginning, middle and end.
Write your final draft, proofread it for spelling and punctuation errors and make any last changes.