Crush strawberries completely with a potato masher; pour into the crock of a slow cooker. Pour lemon juice over strawberries. Sprinkle pectin over strawberries. Let mixture stand 10 minutes. Stir sugar through the strawberries. Cook on Low for 2 hours, stirring twice while it cooks. Increase heat to High and continue cooking until jam thickens to desired consistency, 2 to 3 hours.