The 5 pdfs files corresponds to slides for each of the sessions of the 5h course on citizen science for the study of complex social phenomena held during the XI GEFENOL Summer School on Statistical Physics of Complex Systems. Barcelona, July 17-28, 2023. The audience were mostly early career researchers. During the course, we introduce the concept of citizen science and reflect on the possibilities to use participatory research practices for a better understanding of several social phenomena from complex systems and statistical physics perspectives. We talk about the different steps where citizens and civil society organisations can be involved from the project definition to the data interpretation and which key data-based knowledges can be obtained both valid from the participants' perspectives and for academic scientific publications. To do so, we will be sharing the specific experiences and particular the research processes behind several citizen science projects that have used complex systems to develop behavioural experiments with game theory in different contexts, dynamical models for pedestrian mobility in neighbourhoods or specific technologies such as a chatbot to better learn about mental health social support networks. We share a large number of experiences from our group OpenSystems from the Universitat de Barcelona.