For the quince jelly, wipe the quinces with a cloth and cut into small pieces with the core and peel. Boil the quince pieces with a little water until soft but not mushy.
Pour the soft quinces with the juice through a moistened cloth overnight and collect the juice in a bowl placed underneath. Measure 1 quart of the juice and place in a wide saucepan.
Mix with the sugar and bring to a boil. Bring the juice to a bubbling boil for about 10 minutes, stirring and skimming off the foam. Make a jelly test. When the juice gels, immediately pour into clean jars and seal tightly.