The idea of writing a book overwhelms many writers who write articles, features, etc. The sheer scale of the enterprise, the number of pages to be written and the research involved, can appear daunting.
I have encountered people who excel in their chosen fields of study and say that they would like to write a book, but endlessly put off the task. The work involved in putting a lifetime of learning and experience onto paper seems too arduous to actually begin.
The first step to write a book is to start small, that is to write articles for journals, magazines, websites, etc. This will give experience and recognition. More importantly, it will give you confidence in yourself. The next step is to write small booklets on specific topics.
When beginning to write a book, it is not necessary to have thought through all that you intend to write about. Take the plunge. Often, a beginning writer thinks that what he knows will fill only a couple of chapters and so he is tempted to wait until he accumulates all the material. The problem is that you will never get all the material unless you actually start. Once you start, the ideas and words will just keep pouring in. Your output will surprise you.
Once you start to write, you need to focus only on getting the ideas in your head onto the paper. Don't worry about aesthetics when framing sentences or choosing the most appropriate words. These act as speed breakers impeding the flow of ideas. The restructuring and rephrasing of what you have written can be done later.
Join a writing club or get in touch with published authors. They can give you valuable tips on writing and help in critiquing your work. Get someone to edit your work. This is essential to weed out spelling and factual errors. Polish your text and prepare the final draft.
The next stage is getting the book published. When you approach publishing houses, you will likely face rejection. Don't let this discourage you. Learn to take rejections in your stride. Perseverance is the key to success in writing. Sooner or later, you will find someone who will be interested in your work.