In this paper, we refer to a socio-technical system as a model for the organisation of production and consumption in a given territory. This work is part of the STEEP team's "Socio-technical alternatives" research axis, which aims to provide tools and methodologies that allow stakeholders in a territory (public authorities, associations, companies, citizens) to deliberate in order to imagine and evaluate together long-term alternatives (two or three decades) for their territory. These alternative socio-technical systems should meet the needs of the populations while being compatible with various local and global constraints and limits (environmental, social). Our work focuses more specifically on a step-by-step tool for designing coherent alternatives.