Pick through your blackberries carefully, remove the stems, and rinse your berries with clean water.  

Place the blackberries into a medium or large saucepan.  Use a large spoon or potato masher to gently mash the blackberries.  Add 2 tablespoons of lemon juice.

Cook the berries over medium heat, stirring often to prevent scorching while the juices are released from the berries.

Once the blackberries and lemon juice have created a significant amount of juice, add the sugar, stirring to dissolve.

Turn the heat up to medium-high, and continue to stir and watch to prevent scorching.

Cook the jam until it has reached the gel stage, which should be approximately 20 minutes or a little longer.

Test the jam to see if it has reached the gel stage by using the cold plate test, or until the temperature reaches the gel stage at 220° F on a candy thermometer.

Ladle or pour the jam into prepared jars or containers.

Preserve the jam by using a hot water bath canning method, or, after the jam has cooled, keep the jam refrigerated or frozen until ready to eat.