To enable two-factor authentication in Twitter, you first need to verify your phone number andemail address. With that complete, follow these steps:

  1. Visit your account settings page.
  2. Select “Require a verification code when I sign in.”
  3. Click on the link to “add a phone” and follow the prompts.
  4. You’ll be asked to enter a six-digit code sent to your phone via SMS each time you sign in to (Note: That doesn’t mean every time you access Twitter, but just when it requires you to sign in.)

Via Lifehacker