Agent-based models have long been argued a useful tool to support policy analysis, variably targeting the assessment of policy design, as well as establishing its performance. Challenging, however, remains appropriate empirical parameterization and validation of such models. This paper contributes to the development of rigorous accounts of policy modelling primarily driven by policy documents in order to develop general conceptual model. Such models can then serve as a basis for early validation by subject matter experts, but more importantly, inform the subsequent inquiry relevant for the parameterization of such models, while at the same time offering the opportunity to detect deviations from regulated practice. Relying on the scenario of organ donation based on the Swedish legislation, we explore the merits of such an approach, and sketch the individual steps from policy documents to conceptual model. Supporting the methodological process, this paper employs the Institutional Grammar 2.0, which offers selected features supporting the proposed modelling approach.