An outline is a map, not a contract. You can change it as you write, but a rough route stops you from wandering for 20,000 words and then discovering you have no book. Some writers outline in great detail and others sketch a page, and both can work. What matters is that you know where the book begins, where it ends and what each part is doing in between. Spend a day or two on it and you will save weeks later.
Outlining fiction
Start with the ending, then work backward. List the major turning points: the event that disrupts your character's world, the choices that make things worse, the lowest moment and the resolution. Many writers use a three-act shape, with roughly a quarter of the book setting up, half building conflict and a quarter resolving it. Then break each turning point into scenes, and give every scene a purpose, such as revealing something, raising the stakes or changing a relationship. If a scene does none of these, it probably does not belong. Keep the outline as a simple list or a set of index cards so you can move scenes around easily. Characters need attention too: write a short note on what each major character wants, what they fear and how they change by the end.
Outlining nonfiction and memoir
For nonfiction, write your central argument or promise at the top of the page. Then list the chapters in the order a reader needs them, each one answering a single question. Add the story, example or evidence you will use to prove each point. For memoir, choose the through-line first, such as recovery, a career or a family secret, then select scenes that serve it. You are not obliged to include everything that happened, only what moves the story you chose. Gather your research, notes and source documents in one folder, and name files clearly so you can find them quickly so you are not hunting for them mid-chapter. If your book relies on facts, quotes or dates, record where each one came from now. Tracing a statistic back through a messy draft is slow and frustrating, and a clear source list also makes fact-checking easier during editing.