The FOSSR Project Handbook provides a comprehensive overview of the FOSSR research infrastructure, outlining its vision, governance, services, and long-term strategy for fostering Open Science in social science research in Italy. The Handbook describes the development of the Italian Open Science Cloud, integrating national nodes of major European research infrastructures (CESSDA, SHARE, and RISIS). It presents the technological architecture, data centers network, and advanced services supporting data access, analysis, and reuse. Particular emphasis is placed on capacity building, training, and community engagement activities aimed at researchers, policymakers, and civil society. The Handbook also details governance structures, work packages, and sustainability mechanisms,serving as a technical and operational reference.