According to the author, Susan Wise Bauer, middle-school students should start with Level 1 of Writing with Skill, which teaches the basic skills. Each level of WWS forms the foundation for the next level. Start with Level 1, and match your progress to your student's ability.
Dr. 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.