At the beginning of the year, model to your students the purpose of a reader's response. Have them only use the illustration as the telling of their favorite part, where most have them have not developed many writing skills at this point. As the year progresses, urge them to add writing into their favorite part, then have them explain their statements.
Reader response's can be done at the end of Read to Self or SSR. They can also be done as an independent activity during centers or during Reader's Workshop.
Remind your students to relate to personal experiences not to just summarize the plot. Reader Response's are a place to connect, remind your students that you want to hear their VOICE in their writing. This response is also a great place to ask questions about the story.