To use application lock, go to
Settings > Security. Select
App Lock and choose passcode. Enter your desired password and submit. Now when you restart application or lock timeout is expired, you will be asked for this passcode.
To enable biometric (fingerprint) lock, enable
Biometrics option in app lock screen.
To change lock timeout, just slide the
Lock Timeout (in minutes) option. A maximum value of 30 minutes can be set.
When lock timeout is enabled, you will be shown lock app button in notes screen. Clicking on this button will reset the lock timeout.