This is an easy step. Just double click the Home Button and slide the task bar to the right like the pictures below. For the iPhone with iOS 5.x can do the same, except instead of sliding once, you need to slide twice to the right. Just follow the screen shot diagram below. Requirements, you need Apple TV 2nd Generation or 3rd Generation with at least iOS 4.4.4
Be sure to Enable Mirroring to ON like on the last image above.
Troubleshoot: If you don’t see the icon with an Arrow inside a TV look alike icon, that means your iPad or iPhone can’t find your Apple TV. Be sure to check that your WiFi is on and that you are on the same WiFi Network. Apple TV does not need to be on WiFi for this to work. I have one of my Apple TV connected via wired instead of WiFi, and the other Apple TV on WiFi and both my iPad and iPhone was able to find either one.