Prepare your material. If you're writing a piece of fiction, compose a short synopsis of the plot, along with descriptions of the characters that include their personality, motivation and background. If you're writing a piece of nonfiction, study your material very closely and prepare notes and citations to back up your assertions. You should also compose an outline of your material, organizing it chapter by chapter to see how it flows from one section to the next.
Find a place to write. If you have a set location dedicated solely to writing, you will get into the habit when you're there. Make sure it has a comfortable place to sit, a flat writing surface and easy access to reference materials such as dictionaries or the notes you wrote in Step 1. Keep it free of anything that might distract you from writing--no TV or video games, and you may even want to keep computers with Internet access out of the way--and make sure it's in a room with a closing door so you can write without distractions.
Devise a schedule for writing and integrate it with the other things you need to do in your life. Writing is a habit, like anything else. If you can think of a good excuse not to write, you can think of a bad one just as easily. Your writing schedule should include at least a few hours a week for writing, during a time when you don't have any other commitments to interfere with it.
Start writing your book. Stick to your schedule and try to get at least a few hundred words on the paper every time you sit down to write. Don't review or change anything while you're writing; it's easy to become discouraged and stop if you're constantly correcting your work. Instead, endeavor to get an entire draft down on the page without stopping. Once it's there in some form, it becomes much easier to proceed.
Wait a few weeks once your draft is done. The time helps you clear your head and restore objectivity. Pretend your book doesn't exist; just set it aside and go on with your life.
Revise your text while you feel ready. Go through it page by page and fix any grammar or spelling mistakes, rewrite passages you don't like and smooth out areas that feel rough or unfinished. This can be the most discouraging part of the process, but don't give up. Go through the manuscript at least twice; if you have literate friends or an editor, ask them to read it and give you constructive notes on ways to improve it.