I've been asked to write a design doc for my project; Samsara even has a structure to follow for these. After I understand how the overall system works, I will tackle. I want to make sure that there's a strong storytelling element in my doc, avoiding as much jargon as possible.
If you want to see a real-life, in-depth example of this, check out my System Design course: System Design Masterclass: Taro Playlists