Cut out the core of the quince and put it in a saucepan.
Pour water over the core in the saucepan, bring to a boil and simmer for 15 minutes.
Cut the cored quince into slices or half slices.
Then pour the hot brew from the core through a fine sieve or straining cloth into a large saucepan.
Add sugar to the brew and bring to a boil while stirring.
Once the sugar syrup boils, add the quince slices, bring it to a boil and simmer until bubbly, stirring occasionally, 5 - 7 minutes.
Let the quince slices in sugar syrup then cool completely to room temperature (at least 3.5 - 4 hours).
Now bring the quince mixture to a boil again, simmer it bubbling for 5 - 7 minutes and let it cool back down to room warm.
Bring the quince mixture to a boil a third time and simmer bubbling, stirring occasionally, this time for about 30 minutes, until the quince slices are soft and the jam is a red-orange or light caramel color. You can still do a jelly test to make sure your jam will be the consistency you want later.
Pour the quince jam (the quince slices and the fruit syrup, which will gel later as it cools) into sterilized preserving jars to the brim, seal the jars, place them on the lids for about 15 minutes, then turn them over and let them cool.