Recipe (sans microwave) in Guardian:
This is a quick guide (for further insight, turn to Pam Corbin's River Cottage marmalade handbook). Arm yourself with some muslin, string, a good heavy-based saucepan and buy or borrow a proper sugar thermometer.
Makes about three jars
400g Seville oranges (about four)
1 lemon
1 litre water
800g white sugar
1 tsp soft dark brown sugar (optional but it makes the colour rosier)
▶ With a potato peeler or sharp knife remove all the orange zest in strips, cut away any white pith then shred the zest and tie it in a small square of muslin. Finely slice the oranges; pith, flesh, juice and all, and the whole lemon, and tip into a heavy saucepan. Add the water and your muslin bag of zest and simmer for about two hours until the pith is tender.
▶ Pick out the bag holding the zest, and leave to drain on a plate. Line a colander with a few layers of muslin, place over a bowl, tip in the contents of the pan, and leave to drip for an hour - you need all the liquid as it contains the vital pectin that makes the marmalade set. You could squeeze any remaining juice from the pith, but it will make the marmalade slightly cloudy.
▶ You should have about 750ml of liquid. Boil it down if you have more, or add water if you have less.
▶ Return the liquid to the pan, add the zest from the bag, and the sugar. Bring to the boil, then quickly simmer until it reaches 104C. Keep the temperature constant for five minutes. A spoonful on a cold saucer should form a crinkly skin after cooling for five minutes. If it doesn't, simmer for a few minutes more; but you may have to settle for soft-set. Switch off the heat, leave for 20 minutes, spoon the marmalade into hot, sterilised jars, seal with cellophane and rubber bands and leave somewhere cool overnight to set to a jelly.
Oranges are not the only fruit
Other fruits give curious flavours to marmalade. Lemon marmalade tastes of sherbet, navel oranges taste like caramel, and grapefruit gains a dark complexity. But they all need a little help to set. What I do is chop (skin, pips and all) a small bramley apple and puree with 500ml water in a blender, then strain and add the liquid (no pulp) to the pot when I cook the fruit. The pectin from the apple will ensure a good set.