Problem
Begin by researching William Lloyd Garrison. You need to learn about the person's life to understand the person's views, but you are not writing a biography.
Identify the newspaper in which this newspaper would have appeared. Use this Library of Congress to find a paper; be sure to choose one that would have actually considered this editorial (for example, a paper in the Deep South would not have published an abolitionist editorial).
In writing the editorial, develop effective arguments for or against slavery from the perspective of this person. Remember: you are writing as a 19th century figure, not from the perspective of a a 21st century student.