Make a shallow cut from the vent (above anus) to the rib cage.  Be very careful not to puncture the intestines. On very small game you can cut through the rib cage all the way to the neck. 

Pull out all entrails.  Check the liver for white spots which indicate disease.  If the liver is clean, place in a plastic bag with the heart and chill as soon as possible.  Wipe the body cavity with paper towels.