Instructions
Complete Hellfire Peninsula dungeons and the PvP daily quests to increase your reputation to "Honored." You can also complete regular quests in Hellfire Peninsula for reputation, if you have any left. The PvP daily quests are given at Thrallmar for the horde and Honor Hold for the alliance.
Visit and right-click Quartermaster Urgronn in Thrallmar or Logistics Officer Ulrike in Honor Hold. The vendor window will appear.
Click the "Next" arrow at the bottom of the window. Right-click "The Flamewrought Key." After you see "You received the Flamewrought Key" in your chat box, go to the Shattered Halls on the middle floor of the Hellfire Citadel. Click the gate and enter the instance by running through the purple skull.