Write down a brief synopsis of the book: a single page covering the story from beginning to end (or the content you wish to cover if you're writing nonfiction). The synopsis shouldn't go into great depth, but rather act as a general guideline for the book as a whole.
Write a one-page description of each of the characters in your book, starting with the protagonist and proceeding to antagonists, supporting characters, and peripheral characters. Focus on their personality and motivations rather than their physical characteristics: they dictate the characters' behavior in the story far more than superficial traits like appearance.
Develop your synopsis into a larger outline: write one paragraph covering every chapter, significant event or section contained in the book. Make sure the events follow a logical structure. The plot requires proper causality--events need to happen for a specific reason and subsequent events need to be consequences of whatever came before them chronologically. Your structure can be looser with nonfiction books, but it still needs to progress in a logical manner which the reader can follow easily.
Set up a location for writing your book: a quiet corner free of distraction where you can work in peace. It should contain a comfortable chair, a clear workspace and a little privacy. Keep it clear of television or video games which could detract from working, though if music helps you work, you might set up a radio or a CD player.
Establish a writing schedule and stick to it. Set at least one hour a week aside to write--more if you can manage it--and set your writing schedule during a time when you won't have more pressing matters occupying your attention.
Write a complete draft from beginning to end. Don't stop to revise or edit; you just want to get the entire book down on the page. It may take some time, and you may run into blocks where the words don't seem to come, but if you stick to your writing schedule you should be able to work through those troubles.
Set the draft aside and let it sit for a few weeks without looking at it. That restores your objectivity and lets you examine the text with fresh eyes. When you're ready, go over the text and perform whatever edits and revisions you feel are necessary. You'll likely find plenty of spots that could use some polishing. When you're finished, repeat the editorial pass a second time; if you have an editor or a trusted friend, let them read the text and provide constructive feedback, then incorporate their comments into your final draft if necessary.
Finalize your text, then contact a publisher about printing it for you.