Currently, almost 8% of the world’s oceans are designated marine protected areas (MPAs), the majority of which are relatively small and under national jurisdiction. Several initiatives are presently underway in international waters to establish large-scale MPAs, such as for the Southern Ocean under the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). By reviewing the MPA initiative in the Weddell Sea (WSMPA), we aim to guide through the planning steps involved in developing an MPA in the high seas of the Southern Ocean in the context of an international organisation, i.e. CCAMLR. We focus also on the associated sciencepolicy discussion process. To this end, we examine the WSMPA roadmap retrospectively from its beginning in 2013 until today. We discuss the individual planning steps and how these have been designed in detail. Throughout, we show that the planning of the WSMPA was based on a collaborative, science-based process that exemplified best practice in applied science. Lastly, we also provide an outlook on the current situation regarding the establishment of CCAMLR MPAs and point out that scientific best practice may not be sufficient to achieve the consensus and political drive ultimately required for the designation of MPAs in the Southern Ocean.