Buy some Large potatoes, set'm on end (like a log) use a VERY sharp knife, and start carving them just like a log.
They're cheap, not to messy, and stay firm enough to get some practice with. When you hack one up to far.......chop'm into thin slices or dice it and fry it with tomorrow mornings eggs.
Then do some more. They work very well for large scale mock-ups as well.
You can also use a banana or large carrots...Cheap and easy