When I teach Main Idea and supporting details, I have the students draw a rectangle in the air with their hands and say "Main Idea". Then, I have them take their arm and act like their pulling down. They do this three times and they say "supporting detail, supporting detail, supporting detail". Does this help? I hope I didn't confuse the supporting detail part!

For mirror with words we say "Main idea is what the story is mostly about. Supporting details support the mean idea. We say this while we are using gestures then of course the Teach/okay! That is where the power is! If you like to sing I often turn it into a song. The kids LOVE IT!!!