Make sure the fruit is firm; over ripe fruit causes jams and marmalades to ferment or go moldy.
Use the correct proportion of sugar to fruit.
Always simmer fruit gently BEFORE adding sugar. this softens the skins and extracts pectin.
Stir until the sugar has dissolved-then boil steadily WITHOUT stirring until the setting point is reached. Allow plenty of space in the pan for the jam/marmalade to boil, without boiling over.
Test early for setting point, for if you boil beyond this the jam/marmalade becomes sticky and spoiled. In the case of some fruits-marmalade in particular-if you boil beyond the setting point you 'lose' the jellying properties of the fruit and it will never set.
Cover jam/marmalade while hot with waxed circles and cellophane covers.
Store in a cool dry place.
With whole fruit or peel do not pot immediately...allow jam/marmalade to cool slightly and thicken, then stir and pot.