How to Approach Designing a FullStack Web App?

I recently started to learn Elxir and Phoenix as my first backend framework. I also have a project in mind.

The issue is I know how to build individual models and wire up everything but feel lost in how to plan and design those components from the ground up.

While learning React this guide https://reactjs.org/docs/thinking-in-react.html helped me a lot in outlining a process to build out a frontend app using react.

Is there any similar guides for starting web apps with backends ? Any framework is fine but guides closer to Ruby are preferred.

I can't find anything on my own since I don't know what to Google.

Merry Christmas too :)