Create A Detailed UML (or software development plan)

I have a small app I want to code, myself. However, I am still very new to planning a project. I'd like to have a development plan, or UML built to help me code it up. My project is to have an app that is written in C#, for Android, is able to connect to a users google calendar, create events, read calendar events, and do it's own functions natively. My goal is to learn from this, so one of the project stretch goals is a clear explanation of the choices made, and the whys. Done is defined as a plan that could be used by me to create this app, with explanations for the logic behind a class, method, or feature is, and why maybe some other tools might be needed or recommended (like why I should do some sections of the project in kotin, or whatever). To win this bid, you must show an example of what I am looking for, and demonstrate that you understand what I want delivered. Feel free to offer suggestions and advice.