Wash the quinces well under running water, use a clean brush to remove all of the fuzz. Then remove the seeds and cut the fruits into cubes.
Arrange in a pot and sprinkle with sugar. Cover with water. Leave them for 2-3 hours, and then place at the bottom of a preheated 356°F (180 °C) oven. Bake and stir.
Optionally, add more water. Once the pieces are well-cooked, switch the oven off. Fill the resulting material into pre-washed and completely dry jars. Seal the jars with caps, turn them upside down and leave them to sit in the still warm oven.
When completely cool, place in the refrigerator or put in a cold area. Wait a few days - only then is the jam ready to eat.