Wipe the quinces well with a cloth. Cut the fruits into quarters and put them in a pan well covered with water. Bring everything to a boil. Cook over medium heat until the quinces are almost falling apart. Cover it all and let it sit overnight.
The next day, scoop the fruit from the juice. Pour the juice through a sieve lined with a double layer of cheesecloth.
Set the juice aside and cook the quinces one more time. Then press them through a sieve and then through a cheesecloth. Add this juice to the juice from the first time.
Measure the amount of juice in a measuring cup and add the sugar. Add 500 grams of sugar to 1 liter of quince juice.
Bring the quince juice with the sugar to a boil. Reduce to about 2/3. Place a saucer in the fridge or freezer and drop a drop of juice on it. If it feels firm, the jelly is ready.
Meanwhile, rinse the jars and lids and the ladle in hot water with soda or put it in a pan of boiling water. Drain the jars and lids. Fill the jars completely as soon as possible, screw on the lid and turn them upside down.