This guide provides topics to consider when initiating or running a citizen science project on alien species or invasive alien species. It includes key recommendations on how to engage with your target audience, plan and design your project, handle data, perform analyses and evaluate your project outputs. It is intended to help anyone set up an alien species citizen science project. Target people include NGO workers, researchers, policy makers, volunteer groups, government agency workers, project managers, invasive species managers, teachers and the general public. Since there are many published guides on citizen science, this guide summarises the most relevant topics, highlights topics that are more relevant in the context of (invasive) alien species, and refers to other sources of information. The guide is translated in 23 languages.