It's always tempting to walk into class and simply announce: "A 20-page term paper on X course concept is due in two weeks." Most teachers who've given such an open-ended assignment have been unhappy with the results because students don't really have much to go on. Instead, write out in detail why this paper is important for students to learn something in your class. Generally, you'll find that as you make the rationale for the task clearer, the task itself becomes more manageable and more obvious.
Perhaps even more important, if you tell students up front what your key criteria will be as you grade the papers, they'll try harder to make the paper meet all those criteria. We recommend explaining your standards for topics and organization, as well as citation format, proofreading, and page format. Typically, a good assignment sheet will fill one single-spaced page. We include more detail on what to include on your assignment sheet under "What makes a good assignment?"