You're going to have to trial and error some until your mind gets ahold of the concept, but it comes down to dappled patches of shadow and light jumbled together.
This book helped me a lot.
There really aren't any requirements for the contest, except that it has been completed within the last two years, and the size requirements, because if you win, your piece is included in a traveling show for the next year. This is actually the only piece I've finished in the last two years (besides some commissioned portraits), so I didn't really have a choice on what to enter. I haven't entered anything in the last two years, and I hadn't started this one with that in mind, I was actually just experimenting with the grey paper because I've always used white paper in the past. And as for having a good handle on charcoal, I feel like a bit of a fraud, because I use the techniques I learned in this book and they're pretty fool proof. I pretty much hated charcoal before I read that book. I only used graphite, but I wanted to do a piece that required really tricking the eye, and graphite is too reflective for that to work, unless you're standing right in front of it. This was actually my first charcoal drawing since high school, ten years prior--now charcoal is one of my favorite mediums. If you like charcoal, you should read that book!
Edit: Oh, and thanks for your compliments and suggestions. :)