Core and cut the quince into big pieces. Quince skin contains high level of pectin, do not peel the quince.
Then use a food processor to chop the quince into desired size for the jam, not too big and not too smooth either, for a chunky texture. I process the quince to the size of half to a full grain of rice.
In a sauce pan, bring the chopped quince and water to a boil. Turn to medium heat, cover the sauce pan with a lid, and simmer the quince for about 30 minutes.
Check the quince and stir often near the end, to ensure the quince sauce is not burned. If the quince sauce is not thick enough, remove the lid and continue cooking while stirring until the sauce is a little thicker than what you desire (it will thin out more later after adding sugar and lemon juice). I like my quince jelly thick, so I let as much water evaporate as possible.
Remove the sauce pan from heat. While the quince sauce is still hot, add sugar and stir to let sugar melt into the quince sauce. Add lemon juice and stir again to combine.
Pour the finished quince jam into a glass jar. Seal and store in the fridge for up to 12 months.