Building an app, website or any digital design directly takes a lot of time and without a clear view of what you are trying to build, you will become lost. In UI/UX design, the first step usually involves a Wireframe, which gives you a rough idea of your design.
Figma, an app specifically used for designing UI/UX applications and systems, provides a very clear guide on how to do this and its importance.
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What is Wireframing? The Complete Guide | Figma
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It must be said, that the above may be overkill for your project. You could do a sketch of your app on a piece of paper and use that as your wireframe.
A Wireframe. Source: Figma | Link