Start by picking out a top-notch product that you feel is in (a) the same vein of material as you are writing and (b) is very well organized and written.
For example, when I wrote a proposal for a race book, I used Hammer & Helm from Green Ronin, one of the best ever race books in my opinion.
Take the basic framework of that book - the chapter designations, the flow of material and so on - as the framework for your outline. Obviously you will have stuff that doesn't fit in there, and will not have stuff that is equivalent of some sections, but you should get a feel of how the sections flow together.
Take that skeleton and describe each section. What you intend to do with the section, how it interacts with other sections, etc. In a big book (like the one I am writing for Paradigm), this can easily become a page per chapter.
Once it all seems to work together, then I start working on the actual writing. I write point-form stuff first (spell ideas for example). Then I stat them out properly (full spell description and spell stats). Then I work on flavor text for them. THEN I write the chapter introduction, so it leads into the material I have just written for the chapter in question.