How do I build a "professional portfolio" without professional experience?

I [f 24, will have M.A. degree in May] am looking to apply to what will effectively be my dream job as a staff writer for the University in which I am currently enrolled, but I'm not sure how to build a professional portfolio.

Any advice?

Some job requirements: - have at least a bachelor's degree - write up emails and newsletters that will be sent out to current students - experience with manipulating photos - research, writing, fact-checking for two of the University's web pages

The application also calls for a writing sample.

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