You can also lock individual Notes from within the app in iOS 9 or later which I find particularly handy. These days I keep a host of info in notes, account numbers, WiFi passwords, pictures of bar codes etc. so I have found this feature really useful.
Open the Notes app in iOS
Tap on the sharing icon in the upper right corner of the note, it looks like a little box with an arrow flying out of it.
Tap the sharing icon.
In the action menu, find and tap on “Lock Note”
Fill out the password and hint that you want to use as the note lock, and optionally keep “Use Touch ID” enabled” and then tap ‘Done’ *
Fill out password to lock notes.
The note will verify that it has been locked with a “Lock Added” message, you can now lock the note by tapping on the lock icon at the top of the note, or by locking the screen of the device as you would normally.
Note has been locked successfully
Once the Notes app has been closed, the device screen has been locked, or the notes have been deliberately locked, any locked notes will remain locked until the proper password has been entered or Touch ID has been used to unlock the protected note(s). And yes, this works to lock any notes at all, whether they’re a standard text note, a drawing or sketch made in Notes app, a checklist, a collection of pictures contained in a note, or whatever else you have kept in the note(s) you want to lock down.
* Be aware that the set password will lock and unlock any of the notes locked within Notes app, each note does not have a separate password. This applies to Touch ID as well.
To unlock Password Protected Notes in iOS:
Open the Notes app and tap on the note that is locked
At the “This note is locked” screen, choose “View Note”
Use Touch ID or enter a password to view the note
Once you’re finished viewing or editing the note, you can lock it again as usual. You can also remove a lock permanently from a note by returning to the Sharing section, and tapping on “Remove Lock”
Finally, it’s worth pointing out that for iPhone and iPad users that have a Mac as well, as long as the computer is updated to the latest version of OS X and using the same Apple ID, then the locked iCloud notes will be accessible from both devices. You can learn more about password protected Notes in Mac OS X here.