This paper describes the complete modelling process for simulation, starting with the identification of agents and ending with a clear and unambiguous description of their behaviour that can be easily translated into each process-oriented simulation platform. It focuses on the definition of agents, a subject which is described rarely. Agents usually "are there", emerging from the things one can see. Defining agents however, determines completely the range and content of a simulation and is not a trivial case. After a deliberate definition of the agents, a natural way to describe their behaviour is presented. These behaviour descriptions match the real process interaction approach. The described method is developed for educational reasons in order to teach discrete event simulation in a non-mathematical way, but is rather complete, coherent and well-defined.