Wash, stem and halve or quarter the apples into a large pot. (Don’t bother to peel or core them.) Add a few handfuls of black currants or cranberries or a few pitted and quartered plums, if you like. Add enough water to just cover them and bring to a boil and simmer for 20-30 minutes, until the apples are very soft. Let the apples cool in the liquid.

Strain the juice into a large bowl or pot, pressing the apples a bit to get every bit out. (Some people don't do this as it can make your jelly cloudy - I don't mind.)

Measure the resulting juice into a pot (this is easy if you drain it into a pot with measurements marked on the side) and add 3/4 cup sugar for every cup of juice. Bring to a boil over high heat and boil rapidly, stirring often, until the mixture reaches 220°F on a thermometer (or about 2 degrees less per 1000 feet above sea level), or until a small amount placed on a cold plate wrinkles when you push it with your finger. It should take about half an hour.

While it’s still hot, pour the jelly into clean, hot jars, skim off any foam that rises to the top with a spoon, and seal with lids. Set aside to cool. (I find that if I use hot jelly and hot jars, straight from the dishwasher, they seal properly and I can store them for a long time, but this is not proper advice - so feel free to follow proper canning instructions.)