Read the manuscript and make notes, including major plot points of the book, the main characters, the goals of the characters and the obstacles they face. Gather extra materials, such as blurbs from authors in the same genre, as well as any awards the book has won (assuming the book is already published and you're writing a cover for a new edition). Plan to work this material between short descriptions of the story.
Write an engaging hook---one or two sentences that will be set off on at the top of the back cover. The hook should be something that catches the prospective reader by surprise, or something that intrigues or shocks them. The idea is to get them to read the rest of the book cover material, so the hook is important. An example might be, "Martin Jacobs, a retired detective, gets a late-night call from his former partner to help investigate a brutal homicide, only to learn the victim was already dead at the time of the murder." This type of hook begs at least two questions. Why would Martin's former partner need him, and how can someone who's already dead be murdered?
Introduce the characters and their goals. Use strong adjectives to describe the book's characters and draw the casual book browser into the lives of the characters in a few short sentences. Create a connection with the characters that keeps the potential buyer from putting the book back on the shelf.
Grab the prospective buyer's curiosity and imagination. The average consumer will spend between five and fifteen seconds looking at the cover of a novel, including the artwork. Write leading sentences that plant questions in the consumer's mind. Draw readers into the story.
Write to a word count of between two and three hundred words. Capture the essence of the story without giving away the ending or any secrets. Write with strong, active verbs. (Think about the movie descriptions in TV Guide.)