TRILLION project proposes an open, flexible, secure and resilient socio-technical platform to foster effective collaboration of citizens and law enforcement officers. Using the TRILLION platform, citizens will be able to report crimes, suspicious behaviour and incidents, identify hazards and assist law enforcement agents through active participation for achieving better urban security management. In the context of preparation of scenarios that will serve the needs for enhanced citizen participation and improved citizen-LEA collaboration, our target is to propose a methodology towards a comprehensive definition of scenarios, accounting for all related aspects, such as technical, social, organizational and regulatory. A particular scenario from the project is used in order to demonstrate the application of the proposed methodology