You can go back and model how to place commas in a series in your modeled writing. At the end of the modeled writing, you should edit the passage with the students. If writing in a spiral bound notebook, students write on the right and illustrate on the left. After 5 pieces are written, students sign up for a conference with the teacher. If the teacher takes the notebook out of the student's hands, then the student will be looking around the room and not be focused on the editing process.
Students can always write another story if they think they are done.
They may also illustrate on the left as well. They always need to use the editor's checklist before moving on. I love my mom. I love my dad. I love my teacher.