Measure out the amount of juice and make note of how many cups of juice you have (I got 3 cups of juice out of this basket of apples). Add the juice to a large pot and bring to a boil over high heat. Reduce heat to medium and simmer, stirring, for about 20 minutes, skimming off any foam, as necessary.
Ladle jelly into clean jars and leave with lids off until almost cool (without moving jars around). When jelly is almost cool, seal with lids. Let stand in a sunny window, undisturbed for 24 hrs. Wax and store.
*Cold Plate Test: Place a plate in the freezer before you start your jelly. Place 1 tsp. hot jelly on cold plate and freeze for 1 minute. Remove from freezer. Surface should wrinkle when edge is pushed with finger. If surface doesn't wrinkle, continue cooking and repeat test every few minutes.
What to do if your jelly doesn't set: Hey, it happens and it can happen for any number of reasons. If you have left your jelly out for 24 hours and it didn't set, simply dump the contents back in to your pot and re-boil it until it sets (use a thermometer to cook to 220° F. or until it passes the cold plate test, as above). If you want to be super-sure it's going to set the second time, you could also consider adding a bit of commercial pectin.
Be sure to read the "Cook's Notes" in the original post, for more tips, options, substitutions and variations for this recipe!