Designing a prototype for a mobile app involves several key steps. First, identify the app's purpose, target audience, and main features. Then, create wireframes or low-fidelity mockups to outline the app's structure and layout. Next, use prototyping tools like Adobe XD, Sketch, or InVision to build an interactive prototype that simulates user interactions and flows. Focus on key user journeys and test the prototype with target users for feedback and iteration. Iterate the design based on user feedback and refine the prototype until it accurately represents the desired app experience. Finally, share the prototype with stakeholders and developers to align on the vision and guide the development process.