This recipe is straight from the Ball Canning "Bible" - whole fresh ingredients and a small tsp of butter (to keep the foam down) makes for a delicious jam spread - I could not keep my family from eating this out of the saucepan as I was ladeling it into the jars.
Place 8-9 clean half-pint mason jars on a rack in boiling water canner. (You can also use a large, deep saucepan of stockpot that is at least 3 inches deeper than the height of the jars). Fill the jars and canner with cool water that reaches the top of the jars. Cover and bring water to a simmer over medium heat. Do not boil.
Prepare 8-9 two-piece lid and screw closures. Set screw bands aside. Place lids in a small saucepan and cover with water. Heat just to a simmer over a medium heat, but do not boil. Keep lids warm until ready to use. Do not heat screw bands.
Measure sugar into a bowl and set aside. (Sugar is added to the boiling jam all at once, so measuring it ahead of time prevents errors in quantity and eliminates cooking delays).
In a colander placed over a sink, wash strawberries in cool running water. Drain thoroughly and, using a strawberry huller or the rounded end of a potato peeler, remove hulls.
In a glass pie plate or flat-bottomed bowl, place a single layer of strawberries. Using a stainless steel potato masher, crush berries and transfer to a 1-cup liquid measure. As you accumulate each cup, transfer crushed berries to a large, deep stainless steel saucepan. Repeat until you have 5 cups of crushed strawberries.
Add lemon juice to crushed strawberries in saucepan. Whisk in pectin until dissolved. Add 1/2 tsp butter to decrease accumulation of foam during boiling process. Bring to a full rolling boil over high heat, stirring frequently. Add sugar all at once and, stirring constantly, return to a full rolling boil that cannot be stirred down. Boil hard, stirring constantly, for 1 minute. Remove from heat and, using a large slotted spoon, skim off any foam.
Fill one jar at a time. Remove jar from canner and empty hot water back into canner. (Do not dry jar.) Place jar on a tray or towel-covered counter and place a canning funnel in it. Ladel hot jam into hot jar, leaving 1/4 inch headspace. Slide a non-metallic utensil, such as a rubber spatula, down between the jam and the inside of jar two or three times to release air bubbles. Adjust headspace, if necessary, by adding hot jam. With a clean damp cloth or paper towel wipe jam rim and threads to remove any food residue. Using a magnetic or non-metallic utensil, lift hot lid from water and center it on jar. Place screw band on jar, and with your fingers, screw band down evenly and firmly, just until resistance is met, then increase to finger-tip tight. Do not over-tighten or use any tools to apply screw band. Return jar to canner rack and repeat until all jam is used.
When all jars are filled, lower rack into canner and ensure jars are completely covered by at least 1 inch of hot water. Cover canner and bring water to a full rolling boil over high heat. Process (continued rapid boiling) for 10 minutes, starting timer only when water reaches a full rolling boil. At the end of the processing time, turn heat off and remove canner lid. Wait 5 minutes, then remove jars, without tilting. Place jars upright on a towel in a draft-free place and let cool, undisturbed, for 24 hours.
After 24 hours, check lids for seals. Remove screw bands and press down on the center of each lid with your finger. Sealed lids will be concave (curve downward) and will show no movement when pressed. Jars that haven't sealed properly must be refrigerated immediately or reprocessed. Rinse and dry screwbands. Wipe jars and, if desired, loosely reapply screw bands. Label jars and store in a cool, dry, dark place (basements work best as they have more consistent temperatures). Use all product within one year for best flavor.