Instructions
Speak to Sorine Jurard in Fort Dawnguard after you have completed the "A New Order" quest. Open your journal and navigate to the "Quests" menu. Select the "Ancient Technology" quest and read the objective to discover the location of the schematic you are searching for.
Travel to the target location, which will be a randomly chosen bandit hideout or Dwarven ruin. Fight your way through the environment until you reach the boss level chest at the end of the level. Defeat the boss guarding the treasure and then open the chest. Pick up the "Enhanced Crossbow Schematic" and your quest log will update, telling you to return to Sorine.
Return to Sorine with the schematic still in your inventory and talk to her to complete the quest.
Talk to Sorine again and select the "What can I do to help?" option to embark on the quest a second time. Repeat the quest a total of six times in order to fully complete the mission.