A teacher at T.'s school was selling green apples from her yard a few weeks ago, trying to get them all off the trees before the deer ate them, so I bought four or five pounds and made green apple pectin stock—basically concentrated cooked apple juice that's high in pectin so it can be used as a base for all kinds of jellies. You can keep it in the fridge for a couple weeks or freeze it or can it and it'll last for months. This stuff is what I use to make wine and tea jellies in the winter.
You chop up the apples—cores, seeds, peel, and all, add water to just cover them, and simmer until they fall apart into mush:
Then you dump everything into a very fine sieve—I use a bouillon strainer (here's an example, since the picture of mine got lost when my computer died last week)—or a huge jelly bag and let it drain and drain and drain:
Then return that golden juice to the pan and boil it to reduce it by about half. That's it. Green apple pectin stock.