"Secret Conversations" in Facebook

1) Launch Messenger and tap the Me tab (this may be your FB profile photo).
2) Tap Secret Conversations.
3) Slide the Secret Conversations switch to the ON position.
A prompt says "This will be the only device you can use to send and receive encrypted messages." You will still be able to use Facebook Messenger on other devices (such as your computer), but Secret Conversations are limited to your smartphone.
4) Tap Turn On to enable Secret Conversations on this device or Cancel to abort the action.

When you enable Secret Conversations on one device, Messenger will automatically close all secret messages on your other devices.

If you disable Secret Conversations, your existing secret messages will remain on the device until you delete them, but you’ll no longer be able to send or receive them or start new Secret Conversations.

I had previously written about How to Encrypt Your Facebook Messages, but have received replies from some readers saying that they didn't have that option in Facebook Messenger on their smartphones. In order to send and receive "Secret Conversations" in Facebook Messenger you must enable that function using the steps listed above.