Make sure your jars and lids are washed and sanitized. Running the jars through the dishwasher is sufficient. You can also boil the jars if preferred. Place the lids in a pot of boiling water for a few minutes to ensure they are cleaned.
Cut oranges in half and then into thin slices, discarding the white center of the fruit. Be sure to remove the seeds as you go.
Place chopped oranges, lemon zest,and lemon juice, and water in a large pot. Set over high heat and bring to a boil.
After the water comes to a boil, reduce heat and simmer for about 40 minutes, stirring regularly.
Once the fruit has softened, raise the heat and bring to a boil for a second time.
Add sugar and stir continuously.
Once the mixture thickens, it can be transferred into the jars.
Using a canning funnel, ladle the marmalade into the jars until all of the mixture has been distributed.
Wipe the rims of the jars, top with the lids, and secure with the rings.
Fill the large stockpot with water and carefully place the jars inside, being careful to not let them touch the bottom or each other (use a rack if necessary.)
Boil for 10 minutes.
Using canning tongs, remove the jars from the pot and allow to cool to room temperature for at least 24 hours before opening.
Once open, store the jam in the refrigerator. Unopened, the marmalade will last for about 6 months.