Interaction Design (IxD) is focused on the interaction of the user with a digital interface. It is primarily focused on the why it works well for the user rather than the implementation or the overall experience.
In contrast, User Experience Design (UXD) focuses on the overall process, and research associated with everything humans interact with. This is not limited to technology and the methodologies used could be applied to almost anything people interact with.
User Interface Design (UI) is the most narrow by definition with it referring to the visual, surface layer, presentation of an interface. This is often what people refer to as the “look and feel” of a digital interface.
One method used for designing an interaction is to start by creating user journeys. User journeys follow the path of a user that is trying to accomplish a task. The designer focuses on the users end goal and writes a brief step-by-step flow or “storyboard” to mimic the users interaction with the interface. This will help flush out the design at a very high level and get the designer thinking about the steps and interactions the user will take to accomplish a task.