She really does mean underripe, very green apples.
The double straining she recommends is good, but a triple strain through a finer mesh would take off some of the grainy edge to the finished jelly. Be sure to adjust the sugar proportionally.
Don’t add as much water as she calls for — the jelly will take too long to boil down and will become sugary.
Yup, sugary, caramel-tinted jelly. I boiled my first batch too long to compensate for the water.
Use Eureka or another variety of truly sour lemon; Meyer doesn’t cut it for this sweet stuff.
Start the recipe in the morning so you use the strained juice the same day; I’m guessing that the longer it sits the more the pectin breaks down.