Slice oranges and lemon into 1/8 inch or less thick slices using a sharp knife or mandolin slicer. You can discard the stem-end pieces.
After slicing, stack fruit slices, and quarter, to make the sections more manageable when serving
Place fruit in a large (at least 4 quart) pot, and cover with 6 cups of water
Bring to a simmer, and let simmer about 35 minutes, until rinds are tender
Stir in sugar, and stir until dissolved
Bring back to a simmer, and continue to cook until mixture reaches about 224º on a candy thermometer – this will be just below soft call stage, this should take between 15 and 20 minutes. Alternatively, you can use the plate test: place a plate in the freezer when you begin cooking. After about 15 minutes, place a teaspoon of marmalade on the frozen plate, and let set for 30 seconds. If the marmalade is a thick, but wiggly gel, it’s done. If it’s thin, put the plate in back in the freezer and keep simmering. Test again in a few minutes.
When the marmalade is done, remove from heat and jar.
To can the marmalade, Sterilize jelly jars and lids. You can do this by preparing a hot waterbacth canner, with jars in it, bring to a boil for ten minutes, and then remove the jars. I usually use half pint jars for my marmalade, since we typically don’t use a large amount at one time.
Fill sterilized jars with the hot marmalade, leaving 1/2 inch headspace.
Wipe rims of jars with a soft cloth to remove any drips, then screw down lids and bands firmly.
Return the jars to the water bath canner, being sure the jars don’t touch each other, and your canning rack is inserted into the bottom.
Make sure the water covers the jars by at least an inch.
Bring to a boil, and boil for 10 minutes.
Remove from heat and let rest for 20-30 minutes.
Remove jars from canner, and cover with a towel. Let sit at room temperature for at least 24 hours, then test for seals, wash jars if needed, and store in a cool place out of direct light.