01 See for example marvelapp.com.
Over the years a variety of apps have been published that allow you to create digital click-models from simple paper prototypes. The flow of your prototyping session is the same as in paper prototyping, but now using a mixture of paper sketches and app magic. 
In a first step, you create hand-sketched versions of all the screens your users will deal with while using the interface. Using a prototyping app, you then take photos of all those screens, define buttons, and link them to matching other screens. After you have finished linking all the screens, you now have an interactive click-model of your interface that you can use for testing or storytelling.
Some of the prototyping apps are amazingly simple to use. Even people with no prior technical knowledge are able to pick them up within 20–30 minutes. Teaching the use of prototyping apps instead of only relying on written requirements can make a huge difference in the way your subject matter experts talk to developers.