I've found the trick is really getting in there and feeling around. You can’t be squeamish at all. Get in there elbows deep if you have to, start moving stuff around, anything the moves, gives, stretches or squishes comes out. After the initial incision, I basically only use my knife to cut what is keeping something from coming out. That way you're not hacking around in there and accidentally puncturing or slicing the stomach, bowels, bladder and whatnot. You don’t have to know the anatomy (although you should and it’s not that hard to learn). Just clean it out. From where the esophagus/trachea comes into the chest cavity all the way to the anus, if it’s not meat and bone, it comes out. If you couldn’t run a ¼ inch rod from where the anus used to be to the cut off end of the trachea in the neck without touching anything, you’re not done.