See Below

Rub quinces of with towel, peal, remove seeds, and chop into small
pieces. Put peel and seed parts into pot. Cover barely with water and
cook for 30 – 45 minutes. Strain and add loquid to fruit; dispose of
solids. Weigh mixture on scale and ready as much sugar on the side.
Cook mixture for 30 minutes. Add sugar. Keep cooking and stirring
until mixtures drips of ladle in wide, heavy drops. Remove foam. (I
put the foam into a seperate bowl and eat it after it has cooled
down. Delicious!) Fill into glasses (preheat or put spoon into glass
to avoid cracking) and close airtight immediately.