Rehearse what you have to say by making an outline or simply listing the points you wish to emphasize. If you have difficulty articulating your thoughts in writing, simplify your language; you can always make your message more sophisticated later (or not). "I want to know what you meant when you wrote (blank). Your letter was confusing because (blank)" is a good foundation on which to lay a lead-off such as "please clarify your request in your letter about. I was confused by the reference to (blank).