Fill canner (or large stock pot) half-full with water.
Bring to a boil. Turn heat to low and simmer.
Wash jars & screw bands in hot, soapy water; rinse with warm water.
Pour boiling water over flat lids in saucepan (off the heat). Let lids stand in hot water until ready to use.
Crush strawberries. Use a potato masher and crush 1 cup at a time.
Measure exactly 5 cups crushed berries into 6-8 quart saucepot.
Measure exact amount of sugar into separate bowl. (DO NOT reduce sugar amount or the jam will not set correctly.)
Stir in 1 box pectin (Sure Jell) into saucepot with the crushed berries. Add 1/2 tsp butter to reduce foaming, if desired.
Bring mixture to full rolling boil on high heat, stirring constantly. (Rolling boil will not stop bubbling when stirred.)
Stir in sugar quickly. Return to full rolling boil and boil exactly 1 minute, stirring constantly.
Remove from heat. Skim off any foam.
Ladle jam into prepared jars, filling to within 1/8 inch of top.
Close jars with lids. Screw bands tightly.
Place jars in canner.
1. Water must cover jars by 1-2 inches; add boiling water if necessary.
2. Cover canner; bring water to a gentle boil.
3. Process (allow to gently boil) for 10 minutes.
4. Remove jars and place upright on a towel to cool completely.
5. After jars cool, check seals by pressing middle of lid with finger. (If lid springs back, lid is not sealed and refrigeration is necessary.)
6. Let stand at room temperature for 24 hours. Store in a cool, dry, dark place up to 1 year. Refrigerate opened jams up to 3 weeks.