Your first step in writing this paper is to review the arguments by Ruth Benedict and the counterarguments by James Rachels. Let's take a look at these.
Ruth Benedict looks at cultural moral relativism from an anthropological point of view, noting that various societies label different kinds of behavior as good or bad. Morality, according to her point of view, is simply the norms of the culture one lives in. Some cultures, she explains, hold that practices like polygamy, torture, infanticide, suicide, and even homicide are acceptable in some circumstances. Others do not. Each culture determines moral standards for itself, and these standards often vary significantly from culture to culture. Therefore, Benedict argues, morality is relative rather than objective.
James Rachels, on the other hand, argues that the theory of cultural moral relativism has some major flaws. He notes that sometimes people overestimate the differences between cultures and fail to see the values that cultures actually share. Further, he identifies a flaw in the theory's reasoning. Just because people believe in different moral standards does not mean that there is not an objective moral standard that all people should hold to (even if they do not). People disagree all the time, and some people are proven right and others wrong. Perhaps some cultures need to adjust their moral standards because they are simply wrong.
When you have finished your review of the arguments, outline your paper. After a brief introduction, you should start with Benedict's position first, and you should summarize her main point and her supporting points. Then turn your attention to Rachels, and show how he responds to each of these points. You can also talk about Rachels's overall conclusion. If your instructor allows this, you might end with a brief conclusion in which you identify the argument that seems most valid to you and the reason for your choice.