A large part of the IEW philosophy is that students learn to write by imitation, so, naturally, you'll need content to imitate. You can pull material from anywhere you like—for instance, Writing Trails, Imitation in Writing, and DK Eyewitness books all provide great source texts—but IEW has published a variety of pre-made lesson plans with assignments to help students implement the writing theory they’ve learned in Teaching Writing: Structure and Style and the Student Writing Intensives.
The Theme-Based Writing Lessons guide kids through writing assignments based on the TWSS methods, while teaching them about a variety of subjects from science and history to literature and economics through interaction with important texts. To be clear, these are not intended for use as a curriculum replacement—you should still have your students cover these topics with other courses—but they are an excellent tool for broadening subject knowledge while teaching writing.
These books cover from K-12, but use the four color-coded "levels" to see which are appropriate for your situation: Tan for primary, grades K-2; Green for grades 3-5; Blue for 6-8; and Purple for 9-12.