Great question. Here’s how I’ve learned to answer it.
First, a portfolio is a container for stories that answer the hiring manager's primary question: "Do you have what I need for my open designer position?"
• Does it tell them how you solve hard challenges?
• Does it tell them how you learn new things & apply what you've learned?
Can the hiring manager, after hearing the story, see you in their open position?
Practice telling the stories to others.
See what resonates.
Understand what the key points and messages are.
Do screenshots help tell how you solved hard challenges?
Do artifacts or diagrams communicate what you've learned?
Because you now know what the essential elements of the story are, it should be easier to craft a case study that tells the story without you being there.
Usability test that case study. Is it working?
What's the best way to collect up those stories and display them, such that a hiring manager can home in on the one or two that best helps the learn what you're capable of?
The designer thinks about communicating their story as a design problem. They approach it with design tools.