Head to Dark Fissure, Fort Istirus, Fort Linchal or Wendelbek. Wait for the Shade of the Revenant (a purple light) to shine down on the altar. The Shade of the Revenant occurs every 8 days starting with the first day of the month. Place your Grand Soul Gems (they must be empty) in the alter and strike it with your Soul Trap spell once for each gem placed inside the altar. Open the altar and retrieve your Black Soul Gems.