The Use cases methodology and initial requirements analysis establishes the methodological and operational foundations for the implementation of high-value use cases within the SAGE project, a cornerstone initiative of the European Green Deal Data Space (GDDS). This report provides a unified framework that integrates technical, legal, and governance dimensions to guide the design, execution, and evaluation of use cases that demonstrate how data spaces can deliver measurable environmental, economic, and societal benefits. By aligning diverse stakeholders on data flows, processing logic, roles, and enabling tools, the deliverable ensures coherent, scalable, and interoperable deployment of data-driven solutions across sectors.