Describe your book in one sentence. Tell what it does. Say what it means. If you cannot do this, you are not ready to do chapter descriptions for a book proposal. Try doing a one-paragraph pitch instead, or decide what you would want the book cover to say or how the publicity poster would look if it were made into a film.
Go back to the outline, if you are writing nonfiction, or to the storyboard, if you are writing a novel. Pull out your favorite idea or event.
Write about that favorite scene or nugget of wisdom you find to be stellar. Once you have written about it for your chapter description, start writing the chapter itself.
Use the description you have written about the best chapter -- the raison d'etre or "reason for being" -- to give your book proposal the oomph it needs to excite an agent and find out a publisher's bottom line. Work until you have a shining example of your writing. Write, critique then write again. Ask someone else to read it and tell you what they think. Then write it again.