To set up a device passcode for your profile so that no one can pick up your NOOK and start using it without first entering the passcode, do this:
1. Tap the gear icon in the Status Bar to open the Quick Settings window.
2. In the Quick Settings window, tap All Settings.
3. In the list of Settings screens on the left, tap General.
4. Scroll down the General settings screen until you see the heading SECURITY SETTINGS.
5. Tap Security.
Your NOOK displays the Screen Security setting.
6. Tap the Screen lock toggle to turn on the screen lock.
A dialog box opens, explaining that your passcode will be required for all profiles on the device and that child profiles will not be able to access content in other profiles once the passcode is set.
7. Tap the Set Passcode button.
A window with a keypad opens.
8. Enter the sequence you want to use as your passcode. Tap the Continue button.
9. Re-enter the passcode to confirm it.