
Unfortunately, I don't have any direct experience with this. I can only go from what other people have said they've included and then successfully received a voucher.
That being said, you'll probably want to include an introduction about yourself and your interests (also include what you have done and/or are researching, how you're doing it and how long) and why you believe that should qualify you to sit for the exam. You'll also want to make sure you say
why you're interested in the CEH (and how you think it may help you further your career/professional advancement, etc).
I think if you cover those things, you'll probably be in pretty good shape.
Good luck!
BillV