Marmalade is good to keep around for many reasons: orange and lemon peels are rich bioflavanoids like hesperidin, which is also sold in supplement form for a variety of health benefits. Also terrific on pancakes or as part of Crepes Suzette, and as a filling for tea cakes, and makes a special gift.
Step 1: Shred, Slice, or Grate
Seville are the original preferred "bitter" oranges used in Scottish marmalade, and have the flavor profile and natural pectin, to make superb marmalade with no other ingredient but sugar, but if you can't find those, any organic oranges will do, though others will be sweeter and less acidic in taste, and may contain less natural pectin. Organic navels are much lower in acid, bitterness, and pectin, so lemons, limes, or grapefruit added, may improve taste and texture.
The reason I emphasize organic oranges is, if they are not organic, fungicides and other things not safe for human consumption, are routinely used on the peels of conventionally-grown oranges, because the assumption is that the peels will be discarded rather than eaten. Whenever I plan on eating the peel of citrus, I take care to purchase organic.
Whether you use organic sugar, pure cane sugar (so as to avoid GMO beet sugar) or some other sweetener, is your call; results will vary. You will need anywhere from half the weight of the citrus, in sugar, to an equal weight, depending on taste.
For shredding or slicing, I use a high-quality manual cone shredder, but a food processor, or a mandoline, will all work, though the cone shredder/slicer and food processer are the fastest, easiest way to process so many oranges at once.
So, shred or grate or slice to your own preferred thickness, about 12 organic oranges, seeds, peels, juice, and all, into a bowl.
Step 2: Simmer Down
Cook on lowest setting, with the lid on, until the peels are soft. This may take a couple of hours, or several, depending on the heat setting. Cooked with sugar, they will be more translucent. Without sugar, they will not look like marmalade yet, but when they are soft, you can add the sugar, and they will then start to darken and become somewhat translucent.
How much sugar to add is a matter of personal taste, and I give the rough guideline of between half the weight of the oranges, to equal weight. It's easier to add a smaller amount at first, and add more to taste, because of course, the sugar can't be removed once it's added.
If you added the sugar after the oranges cooked, you will need to cook a few more hours, stirring a few times during cooking. If your marmalade is not thick enough, take the lid off and let it continue cooking, to reduce and thicken, stirring once in a while. Your nose will tell you how it's doing. The smell that fills the house while making this, is wonderful.
Step 3: Jar It Up
A canning funnel works great to fill clean canning jars with speed and without mess. Otherwise, you can scoop it into any glass container with a lid. Stirring with a chopstick or anything long and thin, will help dislodge air bubbles.
I pack it in jars while warm, because once cool, it will be thicker and not necessarily pourable. Plastic lids are fine because there will be no heating within the jars. Three pint (or roughly 500 grams, or half-kilo) jars seem to hold the batch, for me. You can of course, use more, but smaller, jars, if you can't use up much at a time.
That's it! Once they are fully cool, you can refrigerate one for immediate use, and freeze the others for later, or give as gifts, with the instruction that they must be frozen or refrigerated immediately. Every bit as good as the heat-canned varieties, to my taste, perhaps a bit fresher. Enjoy!