Fill a large pot with enough water to completely cover jars. Heat water over high heat until it comes to a rolling bil.
Fill another pot with water to cover lids and rims, and simmer. DO NOT BOIL, just simmer the lids. If you boil the lids will activate and will not preserve the berries.
Place sliced strawberries in a 6 to 8 quart saucepan. Crush Strawberries using a potato masher.
Stir in lemon juice, and slowly add in pectin. Over high heat bring the mixture to a rolling boil, while stirring constantly.
Add in sugar and stir to dissolve. Bring mixture back to a boil. Boil for 1 minute, while stirring. Remove pan from heat. Skim foam off top if desired.
Remove Sterilized jars from water .
Ladle or carefully pour hot jam into hot jars leaving ¼ inch at top (headspace). Wipe the rims with a clean towel and put dried lids on jars. Carefully screw the rims on until tightened.
Process jars in a pot of boiling water for 10 minutes. Remove and allow jars to cool on a towel. Once the jars cool, the will vacuum seal. You will hear the lids pop or "ping" once the have sealed. Check lids after 24 hours and make sure they seal. If the lids flex up and down when gently pressed, they did not seal. Store those jars in the refrigerator for up to 3 weeks.
Other jars can be stored at room temperature.