Writing a philosophy paper will probably be quite different from any other paper you've ever written.
A philosophy paper is about presenting an argument. Argue for or against something, and support your position with logical reasons.
Pretend the person reading your paper will be a reasonably-intelligent person who does not know any philosophy, not your professor. The point is to demonstrate what you know, not what your professor knows.
In philosophy, the structure of an argument is important. Before writing sentences, thinking about the structure of your paper. What is your thesis? What points will you use to support your thesis? Create an outline before starting to write and use it to guide your writing. Your reader will be able to understand your argument much easier if the structure is obvious.
Write a draft of your paper. Let others read it. If they do not understand your argument, then you need to rewrite it. The goal is for a non-philosopher to understand your arguments. Though your professor may be an expert on the subject you're writing on, s/he is looking for evidence that you understand the subject and your argument.
Principle of Charity
When attempting to understand the viewpoints of others, if something is open to interpretation, choose the strongest possible interpretation. This usually ends up working out in your favor.
Once you have the basic argument, work on making your explanations complete but concise, and your grammar correct.
Avoid falling into these traps: