Wash the quinces, and cut into chunks - about 1" / 2.5cm cubes.
Add the quinces and water to a saucepan, and heat until simmering. Cook for about 30 minutes, until the fruit is a soft pulp.
Spoon the mixture into a jelly bag suspended over a bowl. Collect the juice that drains off. For the clearest results, don't poke the jelly bag!
Weigh the juice. Weigh out 75% of the weight of juice in granulated sugar.
Add the juice and sugar to a large saucepan - the saucepan should be less than half full. Heat gently, stirring all the time, to dissolve the sugar.
Place the jam jars in a roasting tray in an oven set to 140°C / 275°F / GM 1
Turn the heat up high to until the jelly is at a rolling boil
Start to test for setting either by temperature (when jelly reaches 105°C), or by the wrinkle or flake method.
Once the jelly has reached the setting point, remove from the heat. Fill the jam jars: I spoon the jelly into a glass jug, and then pour into the jars using a jam funnel. Close the jars with lids.
As the jelly cools, the lids will pop showing that a proper vacuum has formed.