Wash the oranges and lemons thoroughly. Remove any stems and cut the fruit into thin slices, removing the seeds as you go.

In a large pot, combine the sliced oranges, lemons, and water. Bring to a boil, then reduce heat and simmer for 30-40 minutes, or until the fruit is soft and easily mashed.

Use a potato masher or immersion blender to break down the fruit until the desired consistency is reached. You can make it as smooth or chunky as you like.

In a separate bowl, mix the sugar and pectin together. Gradually add the sugar-pectin mixture to the fruit, stirring constantly to prevent clumping.

Bring the mixture back to a boil, stirring constantly. Once it reaches a rolling boil, continue cooking for 5-10 minutes, or until the marmalade reaches your desired thickness. Perform a cold plate test to check the set: place a small spoonful of marmalade on a cold plate and let it cool for a minute. If it forms a skin and wrinkles when pushed, it's ready.

Carefully ladle the hot marmalade into sterilized jars, leaving a 1/4-inch headspace. Wipe the rims clean, place the lids and bands on, and process in a boiling water bath for 10 minutes to seal.

Remove the jars from the water bath and let them cool completely. Check the seals, then store in a cool, dark place for up to one year. Refrigerate after opening.