It's a traditional frock coat. They all, unfortunately, look like that in the back. If it helps, think of it without the slit there and think of it as a dress (not a far stretch for Ciel). The top of it will fit you more snugly, going into the bottom which will flair out. There's more fabric at the bottom than at the top, creating that look. Technically that black thing in the back would be holding the extra panels so they didn't flop loose and actually look like a skirt. If you make it more than one piece, it's gathered rather than pleated. Gather it around where the black thing in the back is to hide your stitches.
My best idea for an instructional is to say look into frock coats for how to do those. I'm sorry I can't tell you because I'm honestly not sure just by looking. Here is a thread for someone looking for almost the same thing, including the pattern they want to use (http://community.livejournal.com/ste.../1749686.html)
. And here are professional patterns that may help if you hunt around or buy one of them (1) (2)