The OED is a historical dictionary, meaning it details the historical meaning of the word over time so you will find more than just the current meaning. For each entry the dictionary aims to show the earliest example of the word in a published work and then a range of examples over time. For this assignment, you will need to locate one of the published sources. To locate a source, click on the source link in the entry:
The OED includes books, articles, and newspapers, so you may need to do some research to figure out what type of source you're looking at. If the OED provides more information about your source, follow those links. Otherwise, you should google the title of the source and the author (if available) to determine what type of source you're looking for.
Sources published before 1924 are in the public domain and should be easier to find online. A few places you might find a digitized source: Google Books, Haithi Trust, Archive.org.