Using a vegetable brush, scrub the fruit with a nontoxic, odorless dish soap and hot water.
Cut off the tops and bottoms of the oranges deeply enough to remove the solid disks of pith and reveal the flesh of the fruit. Quarter the fruit and cut away the center rib. Flick out the seeds with the end of your knife. Using the disk blade of a food processor, finely slice the orange quarters. Transfer the sliced oranges to a large nonreactive pot, looking out for any large pieces that made it past the blade. Return these larger pieces to the food processor with ½ cup of the water and puree using the chopping blade. Add the puree to the pot with enough water to cover, about another ½ cup or so. (Alternatively, you can slice the oranges thinly by hand and add them to the pot with enough water to cover, about 1 cup). Bring to a boil. Remove from heat, cover, and let sit overnight.
The next day, bring to a boil and then let sit overnight again.
On the third day, measure the volume of the cooled and softened mixture (you should have roughly about 3 cups), and return to the pot with an equal amount of sugar and the remaining half cup of water. Bring to a boil, stirring constantly to prevent the sugar from burning. Continue cooking until the gel stage is reached. About 20-25 minutes or so.
Remove from the heat. Allow the marmalade to rest for 5 minutes, giving it an occasional gentle stir to release trapped air; it will thicken slightly. Skim off any foam.
Boil a large pot of water.
Ladle the marmalade into clean, hot (submerge the into boiling water) 4-ounce or half pint jars, leaving ¼ inch of headspace between the top of the marmalade and the lid. (I was able to fill 3 jars). Run a bubble tool along the inside of the glass to release trapped air. Wipe the rims clean; center lids on the jars and screw on jar bands until they are just fingertip-tight. Process the jars by submerging them in boiling water to cover by 2 inches for 10 minutes. Turn off the heat, remove the canner lid, and let the jars rest in the water for 5 minutes. Remove the jars and set aside for 24 hours. Check the seals, then store in a cool, dark place for up to 1 year. (ours didn't last that long!)