We don't currently recommend WhatsApp for secure communications. See here for a full explanation.
If you would still like to use WhatsApp, see our tutorial below and be sure to turn off cloud backups and turn on fingerprint change notifications (see section on Additional Security Settings).
WhatsApp is an application that allows users on mobile devices to communicate with each other using end-to-end encryption. With it, users can securely chat with and call each other, send files, and engage in group chats. Although WhatsApp uses telephone numbers as contacts, calls and messages actually use your data connection; therefore both parties to the conversation must have Internet access on their mobile devices. Due to this, WhatsApp users don't incur SMS and MMS fees.
WhatsApp is owned by Facebook. The app itself is closed-source software, which means that it is very difficult for outside experts to confirm that the company has implemented their encryption in a secure way. Nonetheless, the methods that WhatsApp uses to send encrypted messages are public, and regarded as secure.
Installing WhatsApp on your iPhone Anchor link
Step 1: Download and Install WhatsApp
Step 2: Register and Verify your Phone Number
WhatsApp will request access to your contacts. If you grant this access, WhatsApp will have a full list of your contacts' phone numbers. If you do not grant this access, you can manually add each of your contacts for chat messages, however you cannot make a new call without granting WhatsApp access to your phone's contacts. If you'd like to send photos, media, or files, Whatsapp will request access to these files as well.
Using WhatsApp Anchor link
Another verification method you may want to consider is taking a screenshot of the numbers and sharing it over a secondary secure channel, like PGP-encrypted email.
Additional Security Settings Anchor link
Show Security Notifications
As stated above, if for any reason the encryption key of a contact changes, you may want to be notified of this change.
Also stated above, you'll probably want to ensure that unencrypted backups are not sent to Apple.
Navigate to Settings → Chats → Chat Backup to ensure cloud backups are turned off. Under "Auto Backup" choose "Off":