Sketching the screens
After you've identified the tasks and organized your thoughts, the next step is to sketch the screens you envision needing within your app. By drawing out each task and the business process, you already have enough information about how to get from point A to point B. Sketching the screens will allow you to visualize the journey.
Don't be intimidated by the sketching. You don't have to be an artist to throw a few boxes and lines on paper. To avoid getting into too much detail, use boxes to represent icons and images, and use lines to represent text. You can use a tool like PowerPoint to efficiently draw the sketch as well.
Tip
Focus more on the overall flow of the business rather than the details on-screen. With Power Apps, you can easily adjust later.
Note that you don't have to restrict yourself to drawing just one version of the sketch. Try drawing different versions of the screen. Keep sketching until you run out of ideas.
A good way to get ideas for each screen is to look at the apps you normally use and "borrow" their ideas. These apps are often created by professional designers. You can find customer stories and samples of apps from the Power Apps community that you might look into. App templates also show a great variety of different styles of apps.
Check with actual users to see whether your sketches match their expectations. Showing them the sketches and sharing your ideas can help eliminate any difference in expectations. Ideally, you can use the sketches as a "paper prototype" and have your user pretend to actually use it.