After picking a scenario to prototype, you need to capture all the interaction details for each step the user needs to take in the scenario. You can capture the steps to support in your prototype in a sketchflow. A sketchflow combines a sequence of sketches representing each step of a prototype and the different interactions to connect them.

Having a blueprint will help you as a reference for building your prototype. In addition, it will also be useful when reviewing your progress with the prototype. Since prototypes pretend to look like they work, it is easy to forget some interactions, and having a blueprint allows you to check them more systematically.

Your scenario may have an ideal path representing the steps the user would take in an ideal situation. However, with interactive prototypes, you are not in control of the user actions, and they often don't follow the ideal path. Consider transitions to go back, and try to anticipate some of the places where users may go off track.

Providing a realistic context to the user will allow them to get immersed in the experience in which the prototype is being created. Avoid a placeholder text--no "Lorem Ipsum"--and use meaningful and relevant examples. You can also include additional steps in your prototype as a prelude to providing additional context. For example, instead of starting the prototype by showing your app, you can start prototyping from the app store screen where the user installs your app, if you want to reinforce the idea of a first-time experience.