Textbook writing is a lucrative business but it takes a variety of skills to be successful at it. Perhaps that's why so many educational textbooks are written by co-authors.
To my mind, the best way to write coursebooks is to read the syllabus thoroughly and work certain examples from the curriculum into your writing. Secondly, think of your three audiences: students, teachers and Ministry of Education educators; make sure your work is appealing, educational and follows the guidelines. Finally, approach each exercise or activity like a cryptic crossword puzzle - concentrate upon the particular feature you're teaching (e.g. present perfect tense); flesh it out with the theme or topic of the chapter/unit; and check and double check the answers. Nothing is more frustrating to teachers (and students) than the wrong answers!