PREPARE JAR: Wash one half-pint jar and screw band in hot soapy water; rinse with warm water. Pour boiling water over one flat lid in saucepan off the heat. Let stand in hot water until ready to use. Drain well before filling.
PREPARE PECTIN MIXTURE: Gradually add pectin to water in small (1-qt.) saucepan, stirring constantly. Bring to boil on medium heat; boil 2 min., stirring constantly. Remove from heat.
PREPARE A TRIAL BATCH: Measure jam or jelly, sugar and 1 Tbsp. of the pectin mixture into small (1-qt.) saucepan. Bring mixture to full rolling boil (a boil that doesn’t stop bubbling when stirred) on high heat, stirring constantly. Boil 30 sec.; stirring constantly. Remove from heat. Skim off any foam with metal spoon. Immediately pour into sterilized jar. Cover and let stand up to 24 hours to check set of jam or jelly. Meanwhile, store remaining pectin mixture in refrigerator. Store jar of jam or jelly from trial batch in refrigerator.
PREPARE REMAINDER OF BATCH: If Trial Batch sets satisfactorily, prepare remainder of batch in 6- to 8-qt. stockpot, remaking no more than 8 cups of jam or jelly at one time and using same directions as for Trial Batch with the above-listed measures of sugar and pectin for each 1 cup of jam or jelly. Ladle into sterilized jars, filling to within 1/4 inch of tops. Wipe jar rims and threads. Cover with two-piece lids. Screw bands tightly.
PROCESS JARS: Bring boiling-water canner, half full with water, to simmer. Place jars on elevated rack in canner. Lower rack into canner. (Water must cover jars by 1 to 2 inches. Add boiling water, if necessary.) Cover; bring water to gentle boil. Process 5 min. for jelly; 10 min. for jam. Remove jars and place upright on towel to cool completely. After jars cool, check seals by pressing middle of lids with finger. (If lid springs back, lid is not sealed and refrigeration is necessary.) Discard any leftover pectin mixture.