This guide aims to support research librarians serve citizen science projects with the competencies already embedded in many university libraries; they have long been the hub for knowledge of Open Science, they have a multidisciplinary outreach and organise activities connecting students, faculty and the public. Citizen science has a broad scope, and involve voluntary and active public engagement. Embedding citizen science projects in academia may prove useful for many reasons: expanding and improving current research activities and strengthening the interaction of scientists with the public. Citizen Science belongs to the Open Science domain, and is therefore, perceived as a discipline, where research data are shared openly, with open access to publications and full transparency of data availability. However, in some cases, data use have to be limited to comply with ethical and legal conditions, for example due to privacy concerns. For many scientists, major obstacles to share data openly with citizens are the concern of handling personal data, but also the academic reward system weighing publications over data sharing. The FAIR principles are applicable to data regardless of their public availability. The four elements, Findable, Accessible, Interoperable and Reusable are designed to help lower barriers to access generated research and to facilitate potential new findings by promoting the availability and reuse of data. With this guide, we aim to show how research data management in citizen science can benefit from the FAIR principles. The 9 things of this guide are based on research data management challenges identified for citizen science projects (Holmstrand et al. 2020). Understanding these challenges is an important foundation for guidance provided by the research librarian to any citizen science project manager. The 9 things are structured with the FAIR elements in focus, highlighting practical aspects and benefits of FAIR data in citizen science projects.