Flatten, 3 ft. x 3 ft. maximum size, cut to size.Corrugated cardboard: clean, non-waxed.Containerboard: gray/brownboard (food, beverage)NO pizza boxes or any other food-contaminated cardboard (i.e., milk, orange juice, raisins, etc.).NO tinfoil-coated containers (i.e., potato chips, peanuts, etc.).