For a quick primer on writing across the curriculum (WAC), check out the resources provided by the WAC Clearinghouse. You can peruse their Table of Contents, or jump straight to pages such as, “what makes a good writing assignment?” and “how can I handle responding to drafts?”
The University of Minnesota Center for Writing provides a useful set of resources on teaching with writing. Check out their main landing page or jump straight to sections on assignments and activities or grammar and mechanics.
The Writing Center at the University of Wisconsin hosts a set of robust resources on teaching with writing, including tips for working with multilingual writers and designing plagiarism-resistant assignments.