Good RWD starts with structured content.
You need to know “the shape of the content”.
Design the content first, then build upon it.
“What is the message that needs to be communicated if I was only able to provide the user with unsettled HTML”.
Sculpt the content using basic CSS, using browser dev tools.
“See the reality of the content”.
Keep checking on real devices.
“Like sketching, in the browser”.
Then start expanding the browser window. When it no longer works, there’s your breakpoint.