Set aside 4-5 8-ounce jars along with lids and rings. If canning, fill water bath canner with water, but do not set to boil yet.
Trim both ends from oranges, then score the peel in a few places, and remove peel in pieces. Add to a saucepan and cover with water. Bring to a boil, then reduce heat slightly and boil for 15 minutes. Drain and let cool until easy to handle.
2 pounds navel oranges
Using a spoon or melon baller, gently scrape excess white pith from the peels. Don't remove all of it; you need the pectin for setting the marmalade. Slice the peel into very thin strips (cut into smaller lengths if you like) and place in a large pot such as a Dutch oven.
Segment the oranges over the peel in the pot to capture the juices, making sure to remove any seeds.
Add water and set pot over medium-high heat. Bring to a boil, then reduce and cook until the orange peel is very soft, 20-30 minutes.
2 1/2 cups water
Scoop the orange mixture into a large liquid measuring cup (or work in batches). You should have 4 cups; if you have less, add water to reach 4 cups.
If you are canning, at this point set your canner to boil.
Pour back into the pot and bring back to a boil, then stir in the sugar all at once. Cook, stirring often, until the marmalade reaches 220°F.
4 cups granulated sugar
Remove from heat and skim off any excess foam, if desired. Carefully ladle into jars. Use a damp cloth to wipe any spills from the outsides of the jars.
If not canning, place lids and let cool to room temperature, then store in the refrigerator.
If canning, place lids and screw on rings fingertip tight. Use a jar lifter to transfer jars to boiling water bath canner. Place the lid on the canner and process jars 10 minutes. Turn off heat and remove lid, letting jars rest in the water 5 minutes to prevent thermal shock. Remove jars with the jar lifter and place on a towel-lined countertop. Leave undisturbed for 24 hours, then check for seals, label, and store. Any unsealed jars must be stored in the refrigerator.
Notes
Makes approximately 4 8-ounce jars.
If you have extra that does not fill a jar, pour it into a reusable container and store in the refrigerator.
The marmalade will bubbly vigorously once the sugar is added; make sure your pot is large enough, monitor the heat, and stir often to prevent bubbling over.