To begin writing a book, first create an outline that clearly lays out the beginning, middle and end, including chapter breaks if possible. Each day, start with a modest goal for writing a certain word count, such as 300 words per day. This allows for steady progress even if the writer can't initially devote full-time hours to the project.
After completing the outline, it is important to develop a structured space and schedule for effective writing. For example, choose a time of day to write when your mind is the most amenable to creative output and when disturbances are least likely. Similarly, select a place to write where you can remain isolated, unhindered and can best control any possible distractions. Try to work on the book as many days per week as possible, but allow some limited time off to refresh your energy and maintain mental wellness.
Deadlines are important and should not be allowed to pass without incident. When planning deadlines between the initial writing phase and potential publication, allow plenty of time for multiple drafts because good writing involves rewriting. Approach a few people whose intelligence, honesty and literary skills you trust to read initial drafts and provide feedback. No matter what, accept this feedback as constructive, and don't berate yourself. Failure is part of the process forward.