Summarize:

Encrypted messages are only available on the Facebook Messenger app for iOS and Android. They are not available on the Facebook website or the Facebook mobile app. You can turn any conversation with another person into an encrypted conversation. You cannot encrypt group messages. On iOS devices, you can tap "Secret" in the upper-right corner when starting a new message to enable encryption. On Android, you'll need to enable it after starting a message. You can do this by tapping the person's name at the top of the screen (iOS) or by tapping the ⓘ button in the upper-right corner (Android). You'll be prompted to enable secret conversations for your device. You can only send and receive a secret conversation from one device at a time. This means that once you make a conversation secret, you'll always need to access it from that device. To switch devices, you'll have to start a new secret conversation. Once you enable a secret conversation, the other person will have to accept it. This means they'll need to be using Messenger for iOS or Android as well. The feature may not be available to all users. After they accept, your conversation will be encrypted. You can only attach pictures and stickers to encrypted messages. GIFs, videos, audio, and calls are not supported. Tap the Timer button in the text box to select a timer for the message. This will set the message to automatically delete a certain amount of time after the recipient reads it, providing an extra layer of security. On your conversation list, secret conversations will have a padlock icon next to the recipient's profile picture. You may have multiple conversations with one person, since secret conversations are separate from standard Facebook messages. Secret conversations also have a black background instead of a blue one when opened.
Launch the Facebook Messenger app. Open the conversation you want to encrypt. Open the conversation details. Tap the "Secret Conversation" option. Start chatting with your encrypted conversation. Set a timer for your messages. Identify your encrypted messages.