What do I need to do to write a solid 3,000 word essay for a graduate seminar on Geoffrey Chaucer's The Canterbury Tales?
Unlike undergraduate papers, which are intended to summarize existing knowledge, papers in graduate school should make an original contribution to scholarship. Ideally, every paper you write in graduate school should be something you can present at a scholarly conference and later revise into an article or dissertation chapter.
Given the amount of existing scholarship on Geoffrey Chaucer's The Canterbury Tales, this means finding some one aspect of one of the tales that has not been covered in the published scholarship and doing very meticulous work on it. For example, you could examine the sources of a particular tale, use material in the Public Records Office to reconstruct the context of a particular mile of the route, apply a particular version of cultural theory to a tale, use a medieval rhetorical text such as Geoffrey of Vinsauf's Poetria Nova to analyse the literary and rhetorical structure of a tale, etc.
Once you have decided on your specific focus, you will need to search your university library website for the MLA International Bibliography. Search through this to find and read existing secondary scholarship related to your theme. (Your reference librarian can help you with doing this sort of bibliographic search if you haven't taken a research methods course yet.)
The opening two paragraphs of your essay must either show that your work fills a gap in the existing scholarship in the field or that it takes a new perspective on or disagrees with existing scholarship.