The detailed outline is an outline done as deeply as you can. Chapter titles and three levels of heads within each chapter are par, but I’ve had a few cases where I wanted to go deeper (and did). The more detail you can get into your outline up front, the better estimates you can make and the clearer picture you’ll have of what’s in the book.
List everything in the detailed outline, including every bit of front matter, chapters, appendixes, glossary, index, coupons and ads for the back of the book, and possibly even back cover text.
Use heading styles for your headings, because you can then collapse and expand levels in Word, reorganize, and change your style formatting quickly if you need to.
Put descriptive comments for most headings. Each chapter should have a sentence or two describing what the chapter is about, as should each major heading in the chapter. I use a Heading 7 style for that’s been indented two inches and is in a small font. I could use a text style for this purpose, I’m sure, but I just have gotten used to this over the years.
If you have multiple authors or contributors, each chapter heading should list the name of the person writing the chapter in [brackets]. If a different person is writing an individual section within a chapter, put their name in [brackets] next to the relevant heading. If you have a lot of source material experts involved with the project, you may also want to list their names next to the appropriate chapter titles or heading levels.
The detailed outline is a living document that will change over the course of a project. On my first book, I used the outline as part of my completion ritual: each time I finished a chapter, I replaced the chapter’s outline as I’d planned it with the outline of the actual chapter I was submitting. The actual chapter was certainly more detailed than what I’d scoped out, but it was interesting to see the variance between the two. If you end up with a 75% overlap between what you planned and what you wrote, you’re doing well.