Progress to the later game area where the Witch Crest is obtainable
Defeat the required boss or miniboss guarding the crest
Search the hidden room or reward chest in the boss area
Interact with the crest pickup to add Witch Crest to your inventory
Equip Witch Crest from the crest or charm menu at a bench
If it is tied to a quest, complete the associated NPC request first
If it is missable, reload an earlier save and revisit the location