Open WhatsApp and tap the three dots on the top right of the screen
Hit “Settings” > “Account” and then pick “Two-step verification”
Hit “Enable,” and then pick your six-digit PIN.
This next step isn’t mandatory, but adding an email address will allow you to recover your account if you forget your PIN. WhatsApp will periodically ask you for your PIN while you’re using it so that you don’t easily forget it, but we’d still recommend having a backup.
One more thing: it would be remiss of us if we didn’t mention that, in the past, Facebook (WhatsApp’s parent company) has gotten in trouble for using phone numbers provided for two-factor authentication for ad-targeting. The Federal Trade Commission told the company to stop the practice last year.