The first prerequisite to writing a book is to have something to say that people want to hear. The rest of the process is irrelevant if this prerequisite is not met.
Next you have to be passionate about your topic. If you aren't passionate you will have trouble holding your reader's attention.
There are plenty of people that have written books and articles about writing. Read them if you like. Then, sit down and start writing. Don't worry about starting at the beginning. Start writing about something that you know intimately.
Don't expect what you write to be very good at first. It may be, but it probably won't. At this stage, your trying to find your writing voice. Your writing voice won't necessarily be the same as your speaking voice. It will probably be a little more formal than your speaking voice.
When you have written several pages, try reading what you've written out loud. When what you read begins to flow fluidly, you can conclude that you are beginning to find your voice.
Continue writing on this topic until you run out of things to say about it. Then put what you've written aside. Select another topic and start writing about it. Try to pace yourself so that you haven't completely finished writing about a topic when it's time to stop writing for a while. This will make it easier to pick up where you left off the next time you sit down to write.
After a few days, pick up one of the topics that you set aside and read it again. Keep this draft for reference. Copy it to a file with a modified name, e.g. append v2 to the file name. After reading it, you will be more aware of things you'd like to change to get your ideas across more clearly. It's a good idea to bound your editing sessions to a fixed amount of time at any one sitting. This will allow you to stop editing before you get down to unnecessarily tweeking the text. Rewriting is the soul of writing.
After you have written three or four topics this way you can look at what you've written and begin to see how it fits together. This will help suggest more topics to write about and eventually will lead to the emergence of a structure.
Remember, the introduction should be written last. It's impossible to effectively introduce something before you've written it. Also, don't try to write an outline before you have written at least a significant fraction of your first draft. Until you discover what it is that you're writing about, it is just a distraction to try to organize it.
Try to write the same time every day for the same length of time. Regularity is important to establishing a cadence and will help you get in the habit of writing at least a certain amount every time you write. These sessions add up fast. You'll have a rough draft before you know it.
When you have written and rewritten a significant portion of the draft, you may want to solicit comments from someone you trust. Remember, you are asking for their opinion. When they give it, don't argue with them. Take notes on what they say and thank them for their contribution. Then set you notes aside for a couple of days. Don't try to act on criticism until you've had time to think about it. Don't compromise you're author's integrity. If, after thinking about the criticism, you disagree with it, don't act on it. If, on the other hand, you decide that you do agree with it, edit the suggested changes into the next version of your work.
Writing is like giving birth. You have my very best wishes that you are successful at writing the best book written on your topic.
