The paper should be modeled after a published paper in a scholarly journal related to your interests. It should be accompanied by a prolegomenon describing the targeted journal and the paper itself. In the prolegomenon (3 to 5 pages), you explain how you became interested in the topic, why you think the topic is important, and briefly summarize the argument of your paper. You would also (and this is most important) review two or three academic journals that would consider publishing a paper on your topic; then explain why you chose the targeted journal over the others. You should also attach to the prolegomenon a photocopy of a typical paper from the targeted journal.
The paper itself must make a contribution to knowledge in the field. A review of the literature, for example, would generally not be approved as a Plan B paper. The paper should demonstrate your ability to identify a scholarly topic of interest to the readership of the journal, justify the importance of the topic, and prove a thesis. It should conform in formatting to the guidelines offered by the editor of the targeted journal.
You then defend the paper and choice of the journal in a one-hour oral exam.