Place the bottom board with the screen side up, cardboard down. Place so that only the edges are sitting on whatever base you put it on so that the cardboard can be slid out and back in easily. You can then spray a light coating of cooking spray on the cardboard and slide it under the hive. As mites fall through the screen, they get stuck in the cooking spray so they can be counted. Search "mite count" for more info on that.