Setting up iPhone-based app verification
Now, let's switch over to the iPhone. I'll take you as far as I can given that I don't have a phone number associated with this old, retired phone. I now use it as an iPod touch to do bedtime reading. It rocks for Kindle reading. Let's get started, though, getting you up to speed.
Open your Twitter app, Tap the Me icon on the bottom, and then the gear icon in the middle of the screen.
Tap your user account:
Scroll all the way down to the bottom and tap Security.
Finally, turn on Login verification and follow the prompts.
This is as far as I can take you on the iPhone. The rest should be pretty obvious. Congratulate yourself on protecting your Twitter account. It's important that you do this. Make sure to tell your friends as well.