Your Writers Work profile is designed to put the focus on what really matters: your writing! Thus, the writing samples you use in your portfolio should reflect the skills and interests that you want potential employers to know about. Whatever the genre, style, or subject matter, each sample should reflect your talent as a writer.
If you have previously published work that is available online, you can link to it in your portfolio. If the work is not available online, you can copy it into a new document in the Document Editor, save it, and link to it. You can also copy it directly into the portfolio piece itself.
If you don't yet have anything that constitutes professional writing, that's okay! You can create something specifically for the purpose of showcasing your writing skills. When you're creating a writing sample, focus on the type of freelance writing you hope to do. What type of client would pay for the content, and what would they want to see in it?
You'll need at least 3 items in your portfolio in order to promote your services on the Writers Marketplace. Beyond that, keep in mind that basic members are restricted to 5 portfolio pieces, while Pro members can add as many as they like.