Place the pot over medium heat (without the lid), add the lemon juice and stir a little so that the sugar dissolves. Then blend in a blender to the smoothness that you desire (I blended it so that there are still a few small pieces of quince left). Move it back to the pot, add the water and boil vigorously for about 15 minutes (without stirring). Place a little bit of the jam on a small frozen plate to see if it has thickened enough. If necessary, place back over heat for an additional 5 minutes and check again.