Poke a small hole in a leg and stick an airhose in. Seal as best you can and fill up the turtle. Helps peel some of the skin away from the meat. Other than that, nail them down on their back and start cutting. I'm no pro, we only clean a couple a year, but hope this helps. The airhose cut down our cleaning time from ~45mins to 30 or so.
Also, be careful, even after killing them they will still be able to snap their mouths for an hour or two.
