Go to the Underground Jungle side of the world
Look for a large Aether cave in the Cavern layer
Search near the outer fifth of the world, close to the ocean
Dig down in the Jungle side until you reach the Cavern layer
Explore wide horizontal tunnels in the lower caverns
Check for a large glowing liquid pool with a purple-blue sheen
Use a Spelunker Potion to help spot it faster
Use a Metal Detector or GPS to help confirm your depth and location
In new worlds, search on the same side as the Jungle, opposite the Dungeon
If you cannot find it, keep moving farther toward the world edge in the Cavern layer