The easiest way to ensure you don't write without direction is by preparing a thorough outline of your book. This preliminary task acts as a map as you begin writing your book and serves as a compass when you find yourself lost in the details.
When preparing an outline, consider all the elements your book needs to contain. Think in terms of chapters and sections first and begin asking yourself a number of questions: What would the first chapter be about and what would I title it (if at all)? What should logically follow that chapter?
Continue to consider all these points until you have outlined a basic chapter list in the order in which it should appear in the final publication. With your chapter list prepared, begin to outline, within each chapter, the points that each chapter should cover.
While you will most assuredly run into instances during the time you begin writing your first draft where you will need to clarify information, conducting preliminary research for your book can save you the hassle of having to take time away from your first draft in order to find the information you need. Using your outline, create a detailed list of all the information you'll need to research in order to complete your book. When you have a list of research topics, begin researching each point.
Organizing your research as you go is important to keep your first draft running smoothly. This will prevent you from having to hunt for the research you've conducted once you're in the middle of writing.
The key to writing a successful first draft is to prohibit yourself from editing as you write. Consult your outline and research and begin to write. Ignore grammatical and spelling errors as they occur. Treat your first draft as if your writing is timed and you simply can't go back and correct that mistake you've just noticed. All the mistakes that come about as a result of your first draft will be there when you begin the editing and revising stage.
Perhaps one of the least popular phases of the writing stage, editing and revising your first draft and making it into a viable final draft can seem daunting and, at times, near impossible. The key to making your edit work for you is to read your work as if you are a member of the audience for which you are intending to write. Ask yourself if you would understand and enjoy the writing if you had just purchased the book yourself. Don't be afraid to cut away any extraneous writing. Be stern with yourself, but also keep in mind that your work will never be perfect.
Once you have revised your book for the first time, run through it again until you are satisfied that you have completed the book to the best of your ability.