This is a sizable batch of marmalade. Unless you plan to can it, give it to lots of friends or eat it for every meal you might consider cutting it in half or a fourth. I did process mine in a water canner though I think you could just refrigerate it if you plan to eat it straight away (another reason you might not want a mega batch).
The amount of sugar may be a surprise in the recipe however it functions both to take the tartness down a notch as well as help the marmalade solidify. This version is modified from the original to lessen the sugar a bit, taking out the extreme tartness as well as allowing the marmalade be a bit more solid. If wishing a smaller batch, the recipe ingredients could be halved which would still provide for plenty.
Wash the outside of each fruit (peel is included in the recipe). Cut each citrus fruit into quarters and place in the bowl of a food processor. Pulse until the fruit is in small pieces. Refer to the photo below.
Measure and place fruit into a large, non-reactive pot with lid. Cover with 3 times the volume in water. As an example: I had 7 pints of pulverized fruit/juice and added 21 pints of water to the pot. Cover and let sit overnight (or for 12 hours). It does not need to be refrigerated unless in a very warm climate.
The following morning bring the mixture to a vigorous boil for 30 minutes. Stir periodically to prohibit any burning of the pulpy fruit. Turn off heat, cover and allow to sit 24 hours.
After the 24 hour rest period, bring the mixture to a boil again, this time with the purpose of reducing the volume by 1/3. It will need to boil for a few hours. The color will become more amber as the marmalade reduces.
After the marmalade has reduced and is less watery, add the sugar.
Bring the mixture to a boil and boil until it reaches 220 degrees, (gel or soft ball stage), on a candy thermometer.
If you are refrigerating, spoon the marmalade into the jars and allow them to cool fully before putting them in the refrigerator. If canning, process in a water bath for 10 minutes (or the correct time for your altitude; I did for 15 minutes).