Smart homes improve quality of life by providing various services based on information and communication technologies. However, smart home systems are complicated and user interfaces for the interaction between smart home and user are often not user-friendly, causing potential difficulties and inconvenience for the user. Therefore, in order for smart homes to become user-centered, usability needs to be improved. This study aims to present guidelines for improving the usability of smart homes based on the concept of affordance, which is highly meaningful in user-centered design. To do this, firstly, the affordance factors that could be applied to improve the usability of active devices and user interfaces in smart homes were extracted, secondly, a case study was conducted to analyze the application of affordance factors, focusing on active devices (e.g., control devices and smart appliances) and user interfaces that directly interact with users in smart homes. Lastly, guidelines on the application of affordance factors were presented by combining case analysis results with relevant guidelines. Active devices and user interfaces should provide users with appropriate cognitive, physical, functional, and sensory affordances so that users can use the smart home services easily and conveniently.