Sterilize the jars by washing in the dishwasher on the sterilize cycle or immersing in boiling water for 10 minutes. Place clean jars on a clean towel. Put 2 small ceramic plates in the refrigerator, because you will need them later to test the thickness of the jam.
Wash and hull the strawberries. Mash them with a potato masher. You can decide how fine or how chunky you want them. Measure the sugar, and reserve ¼ cup to mix with the pectin.
Mix the pectin, lemon juice, and ¼ cup sugar in a small bowl or measuring cup.
Place the strawberries in a large cooking pot, and add the pectin mixture. Over medium high heat, bring everything to a full boil that does not go down when stirred. Stir the entire time so the jam does not burn. At this point, add the rest of the sugar, stir well, and bring the jam back to a full boil for one minute. Turn off the heat and test the thickness of the jam by putting a small spoonful on a cold ceramic plate. If it’s thick enough, it’s done. If it’s too runny, add ¼ package more pectin, bring to a hard boil again for 1 minute, and test again with the second cold plate.
Let the jam cool for 10 minutes, then stir to distribute the fruit. If the jam is too hot when you put it in the jars, the fruit chunks will float to the top. Meanwhile, place the Mason jar flat lids (not the rings) in very hot (not boiling) water to get them ready.
Fill your jars, using the ladle and funnel, leaving 1/4 inch of clearance space at the top of the jar.
Wipe the rims of the jars with a clean, damp towel. Place the lids on the jars and secure with rings. Do not screw the rings on too tight.
Process the jars, following the instructions for your water bath canner. This jam requires 5 minutes processing at sea level, 10 minutes above 1000 feet altitude and 15 minutes above 6000 feet altitude.
Remove the jars carefully with the jar lifting tongs, and set them on a clean towel. Leave them undisturbed for several hours or overnight to let them seal. You can tell they are sealed if the center of the lid is depressed slightly and does not move when pressed with your finger. Remove the rings so they don’t rust due to trapped moisture, or if you want to leave them on, at least remove them temporarily to make sure they are dry underneath. The jam lasts about a year in the pantry.