Since you want a rounded pot but the baggie has corners, make sure the corners of the plastic are not stuck inside the tufa - keep pulling the plastic out as you firm the tufa. If the plastic gets stuck inside the tufa, when you remove the plastic it will tear chunks of the pot off with it.
Also in this picture you can see how I fold the top of the bag to the inside. This way you can squeeze the tufa between 2 layers of plastic to keep the shape.
