Step by step
Decide which vault is the decoy
Create a new vault, or use an existing one that you would be comfortable showing to anyone.
Make it believable
An empty vault looks suspicious. Add a couple of dozen ordinary items: scans of documents, receipts, travel photos.
Open the decoy vault's settings
Unlock the decoy with its own pattern, open Vault Settings and choose Set Duress Pattern.
Draw a duress pattern you cannot confuse
It must be different from every other pattern you use, and different enough that you will never draw it by habit. Confirm it.
Check your backups
Make sure the vaults you care about are things you can afford to lose, or that copies exist somewhere outside VaultBe. The duress pattern removes their iCloud backups too.
Memorise, do not rehearse
There is no practice mode. Drawing the duress pattern always does the real thing.
What happens when you draw it
The decoy opens normally. Every other vault's keys, files and iCloud backup records are destroyed. Their recovery phrases no longer work. See Duress Mode for the technical detail.
Removing it
Open the decoy vault's settings and choose Remove Duress Pattern. Setting a new duress pattern replaces the old one.