The Internet of Things pattern language can be a valuable tool for practitioners that want to design an IoT system. It offers them abstract proven solutions based on existing real world uses and, thus, makes working with the large amount of different devices, platforms, technologies, and standards in the field of IoT more manageable. Practitioners can use the pattern language to design an IoT system by starting with any pattern they deem suitable and then by continuing to follow the links to related patterns defined by the pattern language. However, when designing an IoT system, applying patterns in a certain order can be beneficial. It allows practitioners to think through important aspects of the system in the right order to minimize context switching and to avoid having to change previous decisions. Thus, we introduce a pattern-based method for designing IoT systems. It guides practitioners through the steps of designing an IoT system in a sensible order. Based on answers to specific questions asked in each step, it points practitioners to suitable patterns and other helpful tools. The result is a pattern-annotated architecture diagram that can be used as basis for further architecture refinement, as a guide for finding existing solutions, and as input for communication with other involved stakeholders.