Prepare clean jars and keep hot until ready to use. I put mine in dishwasher on dry cycle. Keep lids and flats hot in a boiler.
Wash oranges and lemons and cut ends off. Cut in quarters and remove white membrane from middle and all seeds. Thinly slice oranges and lemons and put in medium saucepan. Add water and bring to a boil.
After comes to a boil, reduce heat to medium low and cover and let simmer about 25 minutes stirring occasionally. Measure out exactly 4 cups of prepared fruit in saucepan. Add fruit pectin and then butter to reduce foaming. Bring to a full rolling boil on high heat stirring constantly. Add sugar and bring to full boil again stirring constantly for exactly one minute. Remove from heat and skim off any foam.
Marmalade should be at a jelly like stage and ready to ladle into jars. If not ready, boil a little longer. Fill jars one at a time within 1/8 inch of top. Add lids and flats and screw tightly. Put jars in hot water bath and bring to a boil. Let boil 10 minutes. Be sure water cover jars by 1 to 2 inches.
After the 10 minutes, remove jars with thongs and place on dry towel away from drafts until they are all sealed. If for some reason one doesnt seal, store it in refrigerator