[See also: How to write the background, theory, methods, analysis and discussion sections. How to write the introduction and conclusion. How to review the literature and how to structure a research paper. How to finish on time and how to reference properly. Part of the Craft of Research series. Full program here.]
The short answer is: one paragraph at a time. Write regularly and in moderation about things you know for people you respect. Write every day, five days a week. Don’t write more than three hours a day. Always decide the day before what you will say; make sure it’s something you know. Have someone who is qualified to tell you that you are wrong in mind as you write. Remind yourself that you really do want them to tell you if you are. Enjoy.