Rinse the berries. If using strawberries, hull and slice them. Weigh two pounds into your pot. Weigh one pound (or one and one-half pounds) sugar into the pot as well.
Mix the berries, sugar, grated apple, and lemon juice in the pot. It should be no more than half full or the jam will splatter. Mash with a potato masher until well mixed and the berries are mashed and juicy.
Bring to a boil over medium-high heat. Use the spoon to skim off the foam that rises to the top. This will keep the finished jam clear. (I keep a glass measuring cup next to the stove and spoon the foam into that). Boil for 15 minutes. Stir frequently until you feel the fruit start to stick to the bottom of the pot, and then stir constantly.
When the jam is thick, drip a small amount onto a chilled plate. Run your finger through the jam. If the juices don't run back together, it's ready. If not, cook for 5 minutes longer and check again. Repeat if necessary. Most berry jams will jell in about 15 minutes, though juicy berries may take a little longer.
Ladle the hot jam into the clean jars. Seal with the lids and ring bands. Turn the jars upside down and let cool completely.
When cooled, turn the jars right side up and press the center of each lid. If it remains depressed, it is vacuum sealed, and you can store the jars in a cool place for up to 1 year. If it pops back up, keep it in the refrigerator and eat within 3 weeks or freeze it to use later.