Middle-school students should start with Level 1 of Writing with Skill, which teaches the basic skills of middle-school writing. Each level of WWS forms the foundation for the next level. Start with Level 1, and match your progress to your student's ability.
Susan Wise Bauer's book, The Well-Trained Mind, 4th Edition, covers this topic extensively. If you have an older edition of the book, the following article adds newer information to the older editions.
Susan Wise Bauer expands on her recommendations for middle school and gives direction for teaching writing at the high school level.