Put jars and lids in a large pot with water. Bring the water to a boil and allow to boil while you make the jam.
To make jam, add berries, lemon juice, and sugar to a large pot. Make sure to have room at the top so it doesn't boil over.
Heat the pot over medium heat, mashing and stirring the berries constantly until they have released their juices.
Turn the heat up to high and boil the mixture rapidly, stirring occasionally.
Boil and cook for 8-10 minutes and begin testing for doneness.
You can test for the gel stage by taking a spoon and dipping it into the jam. Pour the liquid out of the spoon and observe how it drips off. If the liquid drips in individual droplets, it is not done yet. You'll know if it has reached gel stage if the liquid no long drips off of the spoon in droplets, but comes off more as a glob or a "sheet." The temperature of the jam should now be 220ºF.
Once the jam is done, remove it from the heat and immediately begin filling the jars.
Carefully take jars and lids out of the other pot using tongs. Fill each jar leaving at least 1/4 inch of space at the top.
Process the jars for 10 minutes. Allow to cool and store in a cool, dry place.