What you'll need
Download a free authenticator app on your phone, such as Google Authenticator or 1Password
Access to the email address linked to your account (if this is the first sign-in method you're adding)
Steps
Go to your account settings. Click your name in the bottom left corner, then open Password & Authentication under Personal settings.
Click on "Set up" next to Authenticator app.
Confirm your email, if prompted. If this is the first sign-in method you're adding, you'll be asked to confirm it's you. Enter the 6-digit code sent to your email address and select Verify.
Scan the QR code. Open your authenticator app and scan the QR code shown on screen. If you can't scan it, select the option to enter the key manually and type it into your app instead.
Enter the authentication code. Your authenticator app will now show a 6-digit code. Type this into the Authentication code field.
Select "Verify & activate". You'll see a confirmation message once your authenticator app has been added successfully.
Managing your authenticator app
Once set up, the Authenticator app entry will show as active under How you sign in. From here, you can:
Replace it with a new device
Remove it if you no longer want to use an authenticator app to sign in
Tips
Set up backup codes as well, so you can still sign in if you lose access to your phone.
Add a recovery email for an extra way to regain access to your account.
Keep your authenticator app up to date and don't share your codes with anyone.