It is so easy that I made it without intending to.
I had started making boiled cider, which is a very boring name for ... boiled cider. It is apple cider which has been boiled so that it is reduced to a syrup. Apple syrup sounds much more appetizing, so I don't know why they don't call it that.
Boiled cider jelly is apple cider which has been boiled long enough to reduce it to ... jelly.
What happened is that I poured, oh, somewhere between a quart or two, into a saucepan. I brought it to rolling boil, then reduced it to something more like a bubbly simmer. I kept it going there for almost an hour, waiting for it to get syrupy.
Foam starts collecting on the top, which I skim off. It does not appear to be getting much thicker, although it is reducing. Boiled cider is reduced to about 1/7 of its original volume, but I can't tell exactly how much I'm reducing the cider.
Finally, after an hour, it has reduced to a mere inch or so on the bottom of the pot, but it doesn't really seem much thicker. It pours. It may coat the back of a spoon, but not very convincingly.
Once in the jar, though, and a couple hours later, I see I am wrong, for it has become gelatinous.
I have made boiled cider jelly.
Apparently I had passed the 1/7 original volume level and went as far as 1/9 the original volume of cider, at which point you get boiled cider jelly all by itself, with no other additives. Apples naturally contain pectins that will allow it jell.
Put a spoonful of this in your mouth and all you taste is APPLE, in all caps. It's very sweet and intense and delicious. Considering that we've left apple season behind, it's a wonderful way to recapture the taste of autumn.
But how to use it? You can use boiled cider jelly as any other jelly, spread on toast for example, but you can also mix a spoonful in a cup of hot water for some instant hot cider. You can also use it as a glaze for hams. I am looking forward to using it as a kind of apple extract to add extra apple taste to apple pie.