Once you've added a few pages, widgets and links, test out your app in the browser by clicking on the green play button.
Download the Android or iOS player apps to test the app live on your device.
Continue editing until you have your first version of your prototype, then...
6. Validate and get feedback on your concept
An idea without feedback is like a tree without roots. Getting feedback on your prototype is the next step and must come from two sources - your project stakeholders and your users. Get their feedback, then iterate in an agile fashion until you have your final solution.