Methodologically, we suggest that modelling must start by an enumeration of the actors and institutions implied in the exercise of modelling, both the actors and institutions to be modelled (internal), and the modelling scientists, and possibly the interested decision makers and/or stakeholders (external). The aim of this paper is to illustrate this methodology by making a medium sized model. This proposed model objective is to understand the relationships between residential segregation and school segregation, resulting from the interaction between school supply and students' choices. We will first identify the various so--?called actors, their objectives and their resulting point of view on the targeted system. Then we provide a synthesis of these points of view. Finally we will discuss the pros and cons of the proposed methodology with respect to knowledge elicitation potentials.