I don't know about you, but either marmalade is TOO bitter for me, or TOO sweet. A good balance is what my palette prefers. So in messing around over the years (and multiple failures) I finally got a good set, good flavor orange marmalade. The secret.... TANGERINES!
So first you get the following to balance the flavor.
1 Naval Orange (Bitter, but juicy so only 1. If you perfect not bitter do all Cece or Valencia Oranges for this.)
3 Cece Oranges
6 Cup Sugar
Slice all SUPER thin and seed.
Added Slices and Juices into a pot w/ 8 cups of water. Bring to a rolling boil for 8-10 minutes. Remove from heat and add 6 cups of sugar. Stir until dissolved and cover. Allow to sit overnight or at least 12 hours.
Next day place back on heat, bring to a boil then reduce heat to medium low and let simmer for 2 hours.
Bring back to a boil and reduce to medium to medium high heat. You need 20-30 minutes on the stove and do not temp get over 220 degrees (use a candy thermometer for this).
When temp has held for at least a minute, check your marmalade for sheeting on a spoon. If you don't know what this is or how to check that, simply put a little plate in the freezer and drink a few drops of the marmalade on the cold plate. You want the marmalade to stiffen, push with your finger and look for the top of the drop to ripple or have a skim on top that is gel like. If it gets really hard quickly, stir in a cup of water to the mixture or your marmalade will be too hard. If its still syrupy/runny double check its at 220 and then cook for another 2-4 minutes and check again until it sets.
Ladle into sterilized jars, place lids on top and waterbath for 10 minutes to seal your jars.