Decide what your book is going to be about. If you are writing a narrative, decide if the story will be plot-driven or character-driven. If you are writing a how-to book (i.e., a cookbook or a self-help book), decide what your book will focus on. Any book should have an easily defined premise, no matter the genre.
Gather information. Writing requires a great deal of research, even if you are writing a fantasy or sci-fi novel. Learn as much as you can about your subject before you begin to write. Inevitably, you will have to do more research as you write, but the process will go much more smoothly if you are well informed before you start writing.
Write your book. Set aside a portion of every day that you will use for writing and stick to this schedule religiously. Keep writing, even if you don't think what you are writing is particularly good (don't worry, you will clean it up in the editing step). It may sound obvious, but in order to write a book, you must find the time to sit down and write.
Edit your book. Don't just proofread; cut parts that don't make sense and add new information that you may have skimmed over when you first wrote the book. Everything you write should add something substantial to the book. If it doesn't, cut it.
Workshop your book. Show your finished work to other writers, take their suggestions and then edit some more. You do not need to take any advice you don't want to, but if you show your book to a handful of skilled writers and get the same comments, think about taking their advice.