This achievement requires you to complete the Lunar Festival quests that award a Coin of Ancestry in the Undercity, Orgrimmar, and Thunder Bluff. I would advise doing the Undercity either first or last, as you can fly between Orgrimmar and Thunder Bluff rather easily and quickly. Elder Darkcore can be found on the surface level of the Undercity (technically outside of the city itself). Head to the southernmost room in the Ruins of Lordaeron to find him in a small room with Lunar Festival Heralds. Elder Darkhorn can be found above the Cleft of Shadow in Orgrimmar, near the flight path for the city. Elder Ezra Wheathoof can be found in Thunderbluff on the Elder Rise (the northeastern bluff of the city) on the northern side of the bluff. Once you have completed each of their quests and receive the Coins of Ancestry that each provide, you will unlock the achievement known as Elders of the Horde.
Recorded with the Windows Game Bar and edited with XSplit.