Go to the Underground Corruption or Underground Crimson
Defeat enemies there while in the Cavern layer
Make sure it is nighttime in-game
Kill enemies in the Underground Hallow for Souls of Light instead, not Souls of Night
Souls of Night drop from most enemies in the Underground Corruption/Crimson when conditions are met
Use Battle Potions and Water Candles to increase enemy spawns
Farm in a large open area for faster drops