Encrypt a folder

To encrypt files within a folder, with no additional software, follow this procedure:

1 Open Disk Utility, and from the File menu, choose New Image ➤ Image from Folder

2 Choose a source folder, containing the files to encrypt, then enter a name to Save As.
Select Encryption: 256-bit AES encryption, and Format: read/write.

3 Enter a password for the image (DMG) file.
This password is used to encrypt the files and password protect the image.

4 The disk image is created, and a DMG file appears on the desktop.
When complete, the original source folder can (optionally) be deleted. A disk image is created

5 Double click the DMG file to mount the disk image, enter the password when prompted.
Ensure that "Remember password in my keychain" is NOT checked for maximum security(!). Enter a password to access

When the encrypted DMG image is mounted, it will behave just like any other drive - except it's encrypted! Any files that are drag-dropped onto the mounted disk will be automagically encrypted and added to the image. Drag to the trash, or right-click and select 'eject' to un-mount it.

Important:

If you forget/don't know the password, the encrypted disk image cannot be mounted, nor the data retrieved.
If you "Remember password in my keychain" (not recommended), the password can be recovered from there.