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.

Pour into a strainer or colander lined with cheesecloth (or use a jelly bag if you have one) set over a large bowl or pot, and let the juice drain out. Stir the pulp around a bit if you want to hurry it up - but any poking or prodding will result in a cloudy jelly. I don't mind this, but you might. That's cool.

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 candy thermometer, or until a small amount placed on a cold plate and put into the freezer wrinkles when you poke at it with your finger. This should take about 20 minutes.

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.)