Read your book and take notes. As you're reading, identify the theme of the book, the tone and anything special about the way it is written (experimental, stream-of-consciousness, etc.). Pay special attention to changes of scene and ends of chapters. You should have at least one line of notes for each separate scene. Your notes as you are reading should identify who is acting in each scene and what they are doing, as well as what effect this action has.
If you are outlining a nonfiction book, take notes on each chapter separately, starting a fresh sheet of paper with each chapter change. Identify each chapter's main points, any special arguments made to support those points and facts defined in the chapter. Try to end your notes with your interpretation of how this chapter furthers the overall argument made by the book as a whole.
Using a word processor or a clean sheet of paper, write an introduction to your book summary outline. Write down the title and author of the book and any notes about the book that may put it into perspective. (For example, "Oliver Twist, by Charles Dickens, was a novel written in the Realism style that criticized the treatment of children in Victorian England.") Using your complete notes, write a thesis statement summarizing the entire book. ("Oliver Twist followed the youth of an orphan from his time in an orphanage through his time on the streets of London, until he found a happy home.")
For a fiction book, use your notes to summarize the main points of the novel. Don't worry about mentioning every single scene, but do mention every critical plot point, character and subplot. Because this is an outline, you don't have to use complete sentences. You also don't need to use a formal outline structure, unless you have been told to do so for an assignment. The most important thing for an outline is that it make sense for you.
For a nonfiction book, you should include every chapter in your outline. Using your notes, go through chapter by chapter and write down the main points for each. It may be easier for you to use a formal outline structure, but you don't have to. If you do use a formal outline structure, start a new major heading (I., II., III., etc.) for each chapter.
When you've completed your outline, set it aside for a day or two, then read through it to make sure you've mentioned every major point. Correct as necessary.