Decide on the theme of your nonfiction book. Write down as many ideas as you can, to get you going. Even if you know what you want to write about, you cannot write everything on the subject. If you decide on a theme you will find it easier to focus on your book. You could, for instance, write a book about knitting that is based on the history of knitting. This would allow you to bring in many other aspects of knitting, but would give you a framework for your work.
Write something every day. You need this discipline to get your book finished. According to Sylvia Hartmann, you should get the first draft down as soon as possible, not stopping to edit. Editing can come later. Concentrate on your introduction as this section will be important for the rest of your writing and as a selling point to potential agents and publishers.
Study the market. Publishers, these days, expect authors to know the competition, and to be able to explain what makes their work different and outstanding. You will also be expected to be involved in marketing. Research the Writers' Yearbooks, so you approach an agent or publisher who handles your type of subject. Make sure your manuscript is as good as you can make it.