Digital business ecosystem (DBE) is a paradigm that enables developing and monitoring novel business models of collaborating organisations and individuals using ICT as the foundation. Different from traditional online networked models such as manufacturer, retailer, or franchise centred, using a shared digital environment, DBE fosters heterogeneity, symbiosis, coevolution, and self-organisation of its multiple actors, which enables it to span different business domains as well as exhibits diverse interests. For many organisations and individuals, DBE presents a new collaborative approach to leverage offered and desired resources among the involved members to meet each of their goals. As such, it is foreseen to be of high value to the involved actors, but at the same time, it is often complex due to many correlated interactions of these actors and thus difficult to design and manage. Furthermore, the current state of the art shows a lack of methodological guidance for DBE design. We propose a method for DBE design that follows the requirements collected from industry experts and practitioners by applying situational method engineering to enable its modularised design. The method for design is validated by action research in the setting of Digital Vaccine, a Swedish DBE managing health-related services.