I used this recipe, from the National Center for Home Food Preservation at the University of Georgia, but substituting all oranges for the recipe's mixture of orange, grapefruit, and lemon. I used seven tennis ball-sized sweet navel oranges.
Traditionally marmalade was made with sour, bitter Seville oranges, but they're nowhere to be found, and I don't miss the aggressive sharpness of such fruit in my marmalade. I've done pink grapefruit marmalade, which is excellent, but today, grapefruit are very, very sweet, too.
Fruit is boiled for about an hour to soften the fruit and remove bitterness from the peel, seeded, chopped, mixed with sugar and water, and boiled for about 20 minutes, until the mixture reaches 220°F. It's then put up in sterile jars, though I did this batch as a refrigerator jam, so no boiling water process after jarring. Once cooled, the jars will go right into the fridge. Got a bit over a quart of sweet orange marmalade.
Fruit is boiled for an hour to soften the peels, and remove a good deal of the bitterness.
Fruit is cut up, seeded (if necessary), and chopped fine.
Finely chopped oranges.