Based on the experience gained during the project, a guide was prepared for registries that plan to join the project. FAIRVASC Onboarding Handbook introduces the background and aims of the FAIRVASC project, a description of the concept of the Semantic Web and FAIR Principles, and practical “how to” guidance for onboarding of a registry to the FAIRVASC consortium. It is aimed at both registry clinicians and computer scientists and requires no previous knowledge of Semantic Web technologies or rare disease research.