Trim both ends of the apples, but keep the peel and seeds, which contain a lot of pectin.
For a gorgous jelly, do not stir while cooking. Press the apples down with a spoon, if necessary.
While it is tempting, do not squeeze the fruit. The pulp could then pass through the cheesecloth (butter muslin) and the jelly would not be perfectly clear. If the amount of juice obtained is different than what it takes, use 180 ml (3/4 cup) of sugar per cup of juice.
Unsterilized jars will keep for several weeks in the refrigerator.
Change the flavour!
To flavour the jelly, you can add 2.5 ml (1/2 teaspoon) of ground cinnamon while cooking. Flavoured with fresh mint or rosemary, this jelly is delicious served with pork or lamb.