Companion Modelling (ComMod) is an iterative approach of modelling in interaction with local stakeholders. KatAWARE is a multi-agent model constructed through a ComMod process with the Water User Association of the Kat River Valley, Eastern Cape, South Africa. By describing the construction of the KatAWARE model, the aim of this paper is to propose a detailed methodology to formalize and to systematize the modelling phases of Companion Modelling. The Kat River catchment will serve as case study for the application of the proposed methodology. This methodology is composed of four steps: (1) the specification of the structure of the system, its dynamics and the indicators one wants to monitor, (2) the description of the initial state of the simulation, (3) the implementation of the model which can take the form of a computer program or of a role-playing game, (4) the reflection step to criticize the model and to propose further improvements. For the first two steps, we propose to use a representation based on the Unified Modelling Language.