Rinse the mulberries thoroughly, discarding any stems or debris, and transfer them into a medium saucepan. Warm over medium heat while gently mashing the berries to encourage the release of their juices.

Once the mixture reaches a boil, stir in the granulated sugar and lemon juice. Lower the heat and continue stirring until all sugar crystals are dissolved.

Return the mixture to a rolling boil, then incorporate the liquid pectin. Stir frequently to prevent sticking, and allow the jam to simmer until it begins to thicken slightly.

As the jam simmers, sterilize the mason jars by boiling them in a large pot.

When the jam has thickened and reaches a final boil, promptly ladle it into the prepared jars, seal tightly, and allow to cool.