To enable the Talkback function, go to Settings -> scroll to the bottom -> tap Accessibility -> Tap TalkBack -> Tap On in the top right and tap OK.

There is a built-in tutorial that will tell you how to use the tablet once the TalkBack function is turned on.

In short, buttons will require a double-tap, as will launching apps. Scrolling will require two fingers instead of the usual one. Sliding down the notification and settings pulldowns from the top of the screen will require two fingers as well.

Double-taps have to be done quickly, else they will register as a single tap.

Talkback mode will render the stylus unusable in situations that require scrolling or sliding, though you can still use it to double-tap on buttons and apps, and it should still work just fine in the Tegra Draw, Write and other stylus-enabled apps.

To disable TalkBack, double-tap the apps tray on the hotbar, two-finger scroll to the Settings app and double-tap it, two finger scroll down to Accessibility and double-tap it, double tap TalkBack, double-tap the On button in the top right, then double-tap OK. This will  turn TalkBack off and restore the tablet to normal function.