Start by setting up a quiet space to write--a study or some similar location in your home, free of distraction and ideally with a door to close. Perform any research required on the subject of your book. If you intend to write a piece of nonfiction, thorough research (and proper citation) is vital. Even if you're writing fiction, it pays to bone up on any particular topics involved: knowledgeable details lend your story a sense of plausibility. Finally, write up a basic structure for your book, covering the narrative arc, chapter organization and basic themes you wish to cover.
Writing the first draft depends solely on your own sense of pacing and the writing techniques that work best for you. The key is to get into the habit of writing regularly. Establish a set "writing time" during a particular hour of the day or day of the week and stick to it. Have a writing goal for each session--even if it's just 500 words--and don't stop writing until you reach it. You must write a book as if you're eating an elephant--one bite at a time; a steady routine enables you to do that.
Some writers like revising page by page or chapter by chapter as they go along, but for many writers--especially first timers--the best thing to do is just power through until you have a complete draft done. Once it's finished, set it aside for a few weeks, then return to it and start to make revisions. Polish the text, tighten up passages that don't work, drop sections you feel are extraneous and make sure the book flows smoothly from one chapter to the next. At least two revisions are recommended, though you can certainly do more if you are so inclined. You may also want to show the draft to another person--a spouse or a trusted colleague like your editor--and ask him to provide comments. He often possesses objective clarity you may lack at this stage. Once the revisions are complete, you can then go about finding a publisher.