Read up on the two points of view. Since almost everyone in the 21st century is anti-slavery, you should read older sources, such as pre-Civil War newspaper editorials, political speeches and other publications in support of slavery. You can find anti-slavery points of view from various time periods, and you should make sure you keep their historical context in mind--1860s anti-slavery arguments tend to be very white supremacist in nature.
Jot down a few key points to focus on. Since you are writing a one-page essay, you should only focus on two or three points that highlight the similarities and differences between the two points of view.
Write an outline following the compare/contrast format. The first paragraph should discuss the pro-slavery point of view, while the second paragraph discusses the anti-slavery point of view and the third paragraph examines the similarities and differences between them. Write these with headings that will turn into topic sentences and subheadings that will turn into explanations and examples.
Write a rough draft, using the outline as a guide. Again, the headings should change into topic sentences, while the subheadings should turn into explanations and examples. Open your first paragraph with "pro-slavery views are rare in the 21st century, but this was not the case in the 1860s," then move into a few reasons why this was the case and some examples.
Edit your essay and make any necessary rewrites for spelling, grammar and general clarity.