An apparatus for assembling a liner from a plurality of longitudinal sections of flexible material having longitudinal edges adapted to be joined one to another to form the liner includes a path means along which the longitudinal sections of flexible material can be conveyed in a laterally reduced condition, an assembly zone at which the longitudinal edges can be joined together to form the liner, and a guide structure about which the flexible material can turn upon exiting from the path means to provide an inner section and an outer section turned back with respect to the inner section. The guide structure presents a guide, surface over which the longitudinal sections of flexible material can pass, the surface being configured to facilitate spreading of each longitudinal section from the laterally reduced condition.