Dry ice is frozen carbon dioxide. You need to let it change from a solid to a gas (sublime) outdoors. Put it in a plastic container and place the container in an open air location or in a well-ventilated area (outside is best) where no build-up of vapor can occur. Pick somewhere that is not accessible to people or pets. Depending on the amount of dry ice and the temperature, sublimation may take a few hours to a day.