If you haven't started yet, or have made a start but are struggling the one piece of advice is to plan your book thoroughly with a chapter plan and stick to it. This then becomes a box ticking exercise and you can steadily work your way through the chapters at your own pace.
My #1 writing tip is, however, to dictate your content and then transcribe it. You speak as quickly as you can think but most people type at half the speed they can type which means you generally can't get your content down quickly enough if you are writing it. See more about my #1 Writing Tip here: