This guide is created within the EU founded CityCLIM project to understand the contributions of citizen scientists to our CityCLIM project, and to guide you on your journey to becoming a citizen scientist. The manual is aimed at citizens who live in the CityCLIM pilot cities (Karlsruhe, Valencia, Thessaloniki and Luxembourg) and want to provide us with weather data measured by weather stations, citizens who use the sensors provided in the project to measure small-scale weather data, or citizens who live in and outside the CityCLIM pilot cities and want to provide us with historical meteorological data. This guide will provide you with information on what it means to be a citizen scientist and the types of data that can be collected in general, and will guide you through the process of becoming a citizen scientist with CityCLIM, from understanding the basics of your possible contributions, to the specifics of the sensors used, data collection and submission. It will also introduce you to the reward and recognition system we propose to thank you for your valuable contributions.