This document presents a comprehensive testing and validation framework for the EOSC Beyond Pilot Nodes network, establishing a federated European Open Science Cloud (EOSC) infrastructure. The methodology encompasses three core components: a Minimum Viable Product defining baseline participation requirements, systematic testing across individual Nodes and integration points, and Scientific User Stories with acceptance criteria ensuring practical research value. Ten Pilot Nodes spanning diverse domains—from social sciences to structural biology, distributed computing to climate science—undergo rigorous evaluation against standardised criteria whilst accommodating domain-specific requirements. Validation operates at multiple levels: Node-level assessment of service registration, metadata provision, authentication integration, and interoperability standards; and integration-level verification of unified access across heterogeneous systems. Results demonstrate technical integration feasibility and confirm researcher value through reduced friction in accessing distributed resources. The best practices establish a replicable model for broader EOSC deployment, supporting the vision of a pan-European open science infrastructure serving all research disciplines.