In response to a request, here is the recipe I use for my Seville orange marmalade. It's relatively simple and quick compared to some of those which require leaving overnight. What you need:
1kg Seville Oranges (or ordinary oranges if you can't get Seville)
2 Litres water
2 kg sugar
1 lemon
muslin or a fresh new j-cloth and some cooking twine
two saucers
a candy thermometer (optional but it does help)
Use a big pan with a thinnish bottom - you need to be able to control the cooking temperature easily so forget cast iron casserole dishes. Put your oranges in the water, bring to the boil and simmer for about two hours, covered, until the skin is soft and you can pierce it easily with a fork. Turn the heat off and take the oranges out. Leave the water there.
When the oranges are cool enough to handle, cut them into quarters and gently scrape all the flesh away from the skin. You need to leave the skin itself, and remove most of the white pith. Separate the pips from the flesh and put them on a square of muslin. Put any juice from the flesh back into the water.
Put your two saucers into the freezer.
Cut the skin into thin strips. Leave in a bowl for now.
Add the juice of your lemon, tie the pips in muslin and dangle it into the water. Bring to boil again for about ten-twelve minutes. Put your candy thermometer in the water and keep your temps at about 220 degrees F.
(At this point, preheat your oven to a very slow temperature, about 140 celsius (not sure what this is in fahrenheit). While this is happening, wash your jars - you'll need approx 8 x 250 gm jars or equivalent. When your jars are washed and rinsed, don't dry with a towel but put them on a tray in your oven for about 10 minutes to sterilise.)
Take your pips out. Now add the orange skin, and boil gently until your liquid is reduced by one third. Tip: use a wooden skewer. Dip it in at the start, and mark the level on the skewer. Keep testing until it's down by a third.
Slowly, slowly add your sugar, stirring all the while to help it dissolve. Now simmer uncovered at 220 for about 20 minutes. Stir occasionally.
After 20 minutes, take a saucer out of the freezer and put a little dollop of marmalade on it. After a couple of seconds push it with your finger. If it has a 'skin' on it which wrinkles when you push it, your marmalade is ready to cool. At the most, you should cook your marmalade for no more than 25 minutes or it'll be really thick and gooey and won't spread well. If you find your mixture doesn't set, you can buy pectin from the healthfood store and add a little bit. Your pips in muslin add pectin to the mixture to help it set.
Let the mixture cool somewhat and spoon it into your jars, and seal immediately. Enjoy!!