Rinse lemons and oranges. Using a vegetable peeler, cut colored part only from 2 of the lemons and 4 of the oranges. Finely chop peel. Put 2 tablespoons lemon peel, 6 tablespoons orange peel, and 1 cup water in a 6- to 8-quart pan; discard remaining peel.
With a sharp knife, cut and discard remaining peel and membrane from the lemons and 10 of the oranges.
Cut fruit into chunks, discarding seeds.
In a blender or food processor, whirl lemons, then oranges, a portion at a time, until you have 2 cups that are smoothly pured.
Pour pure into pan.
Whirl remaining fruit, rub juice through a strainer into a bowl, and measure. Squeeze enough juice from remaining oranges to make a total of 4 cups; add juice to pan.
Place pan over high heat and bring to a boil, stirring often. Continue to boil, stirring often, until reduced to 5 cups, about 8 minutes.
In a small bowl, mix pectin and 1/4 cup sugar.
Add pectin mixture to pan. Stir over high heat until mixture returns to a rolling boil.
Add remaining sugar and stir until mixture returns to a rolling boil. Stir and boil exactly 1 minute.
Remove from heat.
At once, ladle marmalade mixture into clean canning jars (1-cup size) to within 1/8 inch of top. Wipe jar rims clean. Set a new flat canning lid on each jar. Screw on bands.
Protecting hands with pot holders, invert filled jars on a towel for 5 minutes, then turn lid side up.
Let marmalade cool at least 24 hours. Check seals by pressing firmly on centers of the lids. If a lid pops back, it's not sealed; store unsealed marmalade in the refrigerator.
