Password-protect private notes

Add a password

Right-click on the section you want to protect.
Select Password Protection > Add Password.
Type the password you want and confirm it.
Select OK to create the password.

Lock sections

Right-click the section you want to lock.
Select Password Protection > Lock Section.

Or, select Password Protection > Lock All Protected Sections to lock all sections with passwords.

Unlock sections

Select a section that is locked.
Type the password.
Press Enter.

Remove a password

Right-click on a protected section.
Select Password Protection > Remove Password.
Enter the password for the section.
Select OK to remove the password.

Source: Microsoft