Prep: Wash strawberries under cold running water; drain. Remove stems and caps from strawberries. Crush strawberries one layer at a time using a potato masher.
Cook: Combine strawberries, pectin, and lemon juice in a large saucepan, stirring to blend in pectin. Bring mixture to a boil over medium-high heat. Add sugar, stirring until sugar dissolves. Bring mixture to a rolling boil that cannot be stirred down. Boil hard for 1 minute, stirring constantly. Remove from heat. Skim off foam, if necessary.
Fill: Ladle hot jam into a hot jar, leaving 1/4 inch headspace. Remove air bubbles. Clean jar rim. Center lid on jar and adjust band to fingertip-tight. Place jar on the rack elevated over simmering water (180℉) in boiling-water canner. Repeat until jars are filled.
Process: Lower the rack into simmering water. Water must cover jars by 1 inch. Adjust heat to medium-high, cover canner and bring water to a rolling boil. Process half-pint jars 10 minutes. Turn off heat and remove cover. Let jars cool 5 minutes. Remove jars from canner; do not retighten bands if loose. Cool 12 hours. Check seals. Label and store jars.
