The "Best Practices for Collaborative Development of Citizen Science Data Infrastructures" report, conducted as part of Work Package 3, aims to review collaborative initiatives for co-designing and co-developing data services in citizen science projects. The review focuses on lessons learned from past and ongoing projects to establish best practices for different stages of design and development. The report encompasses technical and ethical frameworks, including seven sections and guiding principles. It identifies proven practices and lessons learned while providing key resources for co-designing citizen science data infrastructures and data services. The review includes various aspects of collaborative development, data collection, data standards, data quality assurance, data accessibility, data ownership, policy, and ethics. Special attention has been given to the unique considerations of citizen science initiatives, such as ethics and data privacy. The report is intended for wide dissemination among ECS stakeholders. It serves as a valuable training resource for the European Citizen Science project, supporting the implementation of digital skills, the European Citizen Science Academy, and other training activities.