Wash oranges and lemons. Cut them in quarters lengthwise. Remove fruit from the peel and set aside. Flatten the peel and cut/peel away about half of the white membrane. Cut peels into thin slices (and cut these slices in half if you would prefer a shorter peel in your marmalade).

Place peel in large pot. Add 1½ cups water and baking soda. Cover and simmer for 20 minutes, stirring occasionally. 

Chop the fruit. Stir fruit and juice into the peel mixture. Cover and simmer for 10 minutes.

Measure out 3 cups of cooked fruit/peel mixture, and  discard the remainder. Place the 3 cups of cooked fruit/peel mixture back in saucepan. Add sugar. Bring to a boil on high heat - cook 1 minute. 

Remove from heat and stir in Certo.

Stir marmalade and skim off any foam for a full 7 minutes.

Place in warm, sterilized jars, leaving a ¼ inch headspace. Cover with lids. Allow jars to rest at room temperature for 24 hours.

Store in a cool, dark place for up to one year.