Visualizations have become more complex in recent years so that users need to learn how to use these systems. Onboarding systems can help users to learn how to interact with a visualization efficiently. Simple tutorials are often not sufficient. There are still open questions on how to design such onboarding systems. So far, a few usability studies have been conducted to clarify this issue. We decided to adopt an approach using sketching. We conducted a study with 29 participants. Possible users of onboarding systems were asked to develop prototypes of an onboarding system based on sketches. The process of developing these prototypes and the results were analyzed in a qualitative study. We can show that onboarding systems should follow a step-by-step approach and be integrated into the visualization. In addition, it seems that questions, tasks, and examples should be part of an onboarding system, so that users can try out how to use the system on their own. [For the full proceedings, see ED636095.]