Wash and stone fruit and put into a saucepan with water (under ripe fruit requires more water than ripe fruit).
Cook over low heat until plums are soft. Add sugar and cook, stirring constantly until sugar is dissolved. Bring to a boil and boil rapidly until jam reaches setting point (this takes hours). Pour into warm sterilized jars and seal immediately. Put in canner for 20 mins or so to steralise further.
Makes approx 4 litres.