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.