The Deep Underground Neutrino Experiment (DUNE) will use a large liquid argon (LAr) detector consisting of four modules each with a fiducial mass of 10 ktons of LAr. One of the technology options for the far detector modules is a liquid-argon Time Projection Chamber (TPC) working in Dual Phase mode. A large demonstrator, called ProtoDUNE Dual-Phase, with a 6×6×6 m3(300t) active volume, is currently under-construction at the CERN neutrino platform. As well as being the engineering prototype of a Far Detector module, it will also demonstrate the concept of a very large Dual-Phase LAr TPC. The design of the TPC including the fabrication, testing, installation and commissioning of the various detector components is outlined.