If you have never used a writing activity in your math classroom, this is an excellent place to start. Use a common strategy like think-pair-share, and modify it to include a writing component. This is really easy to incorporate!
This is a great example of how to use writing to wrap up a unit or lesson. In the latter portion of the video, students create a foldable that compares and contrasts linear, exponential, and quadratic equations.
Here you will find an extensive list of ways to use writing in the math classroom, and at the bottom of the page there are some brilliant writing prompts. This is an excellent resource to have open when creating lesson plans.
When you incorporate other subjects into your own, you not only enrich your lesson plans, but also your skills as a teacher. This post does an excellent job of discussing why reading and writing should be used in STEM classrooms, and even gives some very good tips for starting out.