Make a list of things you do well. Everyone is an expert at something. Find your specialty, and you can write many nonfiction books on that topic.
Create an outline for each item on your list. Choose one topic and write down all the key points about it. For instance, a book on nonfiction writing would include key points such as Choosing a Topic, Manuscript Layout, Selling and Marketing, Research Citations, to name just a few.
Flesh out each key point with detailed sentences. Ideally, each of your key points will become a chapter in your nonfiction book.
Add photos, diagrams, and other graphics to enhance your book. Readers of nonfiction love maps, photographs, charts, and other graphics that support your writing.
Add a reference page to indicate where you did the research for your book. Always give credit to your sources. A professional nonfiction book does not overlook the citation of sources.
Put a resource section at the back of the book. Buyers of nonfiction feel they have received an added bonus when they are provided additional resources to find out more information. Go easy on this page though; there is no need to make this more than three pages or so.