Standard jam recipes require one cup of homemade pectin for each cup of crushed fruit.
Thinly slice apples (do not peel or core). Put into a large kettle along with the water. Over high heat, bring to a boil, then lower heat to a simmer. Stir and mash apples occasionally, cooking until very soft.
Pour mixture into a moistened jelly bag or a strainer lined with several layers of cheesecloth and let strain overnight.
Put pulp in a saucepan and add half the juice (store remaining juice in refrigerator; it will be used later). Over low heat, simmer for 1 1/2 hours. Strain through jelly bag a second time. (It might not need to strain as long as the first time.)
Discard pulp and add reserved juice. If using within a week, store in refrigerator. For longer storage, heat, pour into hot, sterilized jelly jars, and seal.