1. Create the Table of Contents -- the table of contents is your book's road map (this mostly applies to nonfiction). Creating your table of contents will give you an idea of how to start. In all nonfiction books the structure should make sense in a kind of "unfolding" way. Each chapter should relate somehow to the chapter before it. For example, your introduction to your topic obviously should be in the front of the book. It's a standard approach. Now I am always for break the rules, but know the rules before you break them.
2. Chapter by Chapter -- You can work a chapter at a time finish and go onto the next one. Don't worry about editing until you have a first draft.
3. Outline the Structure of Each Chapter -- You will want the same road-map approach for each chapter, too. Once you have an acceptable outline back fill the content.