6 cups prepared fruit, (about 4 lbs ripe plums)
1/2 C water
1 box fruit pectin
1T lemon juice
1/2 tsp. butter or margarine, optional (helps reduce foaming)
8 C sugar, measure into separate bowl

Fill boiling water canner with enough water to cover jars with 2" of water, bring to simmer.
Wash jars, lids and bands with hot soapy water, rinse.
Place flat lids in pan of hot water and leave until ready to use.  Drain before filling.

Pit plums. Do not peel. Finely chop fruit. (Just a couple of quick pulses in food processor does the trick.  Place fruit in saucepan; add water and bring to boil. 
Reduce heat; cover and simmer 5 min. (I prefer to not precook plums. I like my jam to be more like preserves)

Measure exactly 6 cups prepared fruit into 6- or 8-qt. saucepot along with lemon juice. Stir pectin into fruit. Add butter to reduce foaming. Bring mixture to full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat, stirring constantly. Stir in sugar. Return to full rolling boil and boil exactly 1 min., stirring constantly. Remove from heat. Skim off any foam with metal spoon. fill prepared jars to within 1/8 inch of top. Wipe jar rims and threads with a damp cloth.  Cover with two piece lids. Screw bands tightly. Process in water bath for 10 minutes.

Turn off heat, remove jars to a draft free area to cool. (I  set jars onto a towel to cool to avoid shocking jars with a cold surface)