Get a sharpie and some paper and get drawing. Screen based apps can be sketched quickly and easily. Use multiple screen frames to indicate the journey through the digital experience from the main screen through onboarding and product use. You can use sticky notes to mock up buttons, sliders and other features of your screens. Walk through various interactions with users in order to gain insights in to natural use. With a member of the venture team, a user and human computer (this person switches screens to meet the commands input by the user) encourage the user to engage with the prototype as they would with a digital version of the app. If a user wants to move a button or slider, or make another change to the prototype, allow them to do so. Be sure to record the change, the reasoning behind it and how it alters interaction with the solution.  Paper prototypes should be used in a Usability Observation setting to get the most valuable and least influenced results.