UML or Unified modelling language is the best tool for modelling object oriented systems. As we know, in objet oriented modelling and design, we identify objects (entities)within the system and analyze how they interact with each other and thus design an interactive model preserving their individual properties, behavior as well as publicizing their public attributes and behaviors. The interaction of objects is of importance rather than just mere data flow as in traditional system development. Object oriented systems supports encapsulation and protection of data and behaviors, better code maintainability and reusability.
Below is an Activity Diagram for Making a customer appointment: