In all my undergraduate English classes, I assign papers. Many professors assume you already know what a good English paper looks like, but I don't take that for granted. I spend plenty of time going over such things in class, but it can't hurt to put everything in writing, too.
Here's the quick version of what I want:
For starters, read my guide, "Getting an A on an English Paper." It contains all my practical advice on what makes a good English paper. Pay particular attention to the section on the thesis, which is the heart of any English paper. I also offer information on how I grade papers.
You should also look at my "Guide to Grammar and Style," which collects all the advice I have to offer on writing. Yes, grammar and such things count, and in my guide, I tell you why.
I assign papers like this not because I like to cause suffering well, I kinda do, but that's not the reason I assign papers because I want to see whether you've acquired certain skills in reading, analyzing, organizing, and presenting information. If you're thinking of writing a radically different kind of paper a "creative" approach to the topic, for instance please please please talk to me about it first. I'm not out to squash anyone's creativity, but I assign papers like this for a reason.
A note on length. When I assign lengths in pages, assume one page is 250 words. You'll get around 250 words to the page if you use Courier, 12-point, double-spaced, with one-inch margins on all sides. I don't insist on Courier, and other faces (like Times Roman) will give you more words per page. I'm not very strict about the exact number of words or pages when I ask for an eight-page paper, I mean about eight pages' worth of thought. If you can squeeze eight pages' worth of thought into five pages, more power to you. If your eight pages of writing add up to only three pages' worth of thought, it's a bad sign.
Don't try to fool me, though, with gigantic typefaces and narrow margins to stretch a short paper into a long one. I'm really not that dumb, appearances to the contrary notwithstanding.
And I've always thought notes like this should be unnecessary, but I've been proven wrong too many times. Let's be explicit: All work should be your own. If anything looks like plagiarism to me, I will spend time trying to find the source. (Professors can recognize plagiarism more easily than you may think.) If I find out any assignment, even a little teensie-weensie one, is stolen, you'll get an F for the semester with no second chance, and I may start disciplinary proceedings to hound you out of school. If I can't find a source and I still suspect the work isn't your own, I'll call you into my office and quiz you until I'm satisfied you wrote it. I take these things seriously. If you want to try sneaking something past me, good luck, but I can't say I recommend it.
If you have any questions, drop me a line.