Although the literature is full of research on the transition of industry towards Product-Service Systems (PSS), the question of how to effectively support PSS engineering is poorly addressed. The compelling need for decision support throughout the various stages of the engineering process is particularly challenging due to the inherent complexity of PSS. In this sense, visualisation and modelling at large have been put forth as promising means for supporting PSS engineering. This paper proposes a method for specifying a modelling language for PSS engineering, putting together PSS domain-specific knowledge and modelling concepts inherited from conceptual modelling and model-based engineering. It relies on a recursive transformation process of the underlying PSS meta-model using knowledge from case studies and the literature. The method has proven to be a practical means for gradual enrichment of the modelling language leading to successful experimentations in the industrial context.