Effective conceptual modelling is critical for accurately representing the physical world in digital twin implementations, yet existing methods often lack comprehensive guidance or overlook key aspects like communication modelling. This research proposes the Entity-Relationship Digital Twin (ERDT) modelling method tailored for industrial digital twins. Building on established Entity-Relationship (ER) models, ERDT extends them with constructs for handling historical data, and defining security interfaces and data flows between physical and virtual components. The method provides a systematic process covering digital twin objective setting, physical component selection, conceptual entity definition, interface specification, and communication modelling. It emphasizes principles like encapsulation and separation of concerns to enhance maintainability. Evaluated against ISO 25010 quality criteria, ERDT demonstrates strengths in functional appropriateness, usability, portability, and maintainability while identifying opportunities for further improvement. A logistics case study illustrates applying the method to a real-world industrial scenario. By bridging conceptual modelling with software engineering best practices, ERDT facilitates developing robust, maintainable digital twins.