1. Open Disk Utility (Spotlight → “Disk Utility,” or Applications → Utilities).
2. From the menu bar, choose File → New Image → Blank Image.
3. Give it a descriptive name and choose where to save it.
4. Set a “Size” large enough for your needs — you can always create a bigger one later if you run out of space, though the container itself doesn’t dynamically shrink once created.
5. Under “Format,” HFS+ or APFS both work fine; APFS is the more modern choice on recent macOS versions.
6. Under “Encryption,” select “256-bit AES encryption” (the stronger of the two offered options — 128-bit is faster but less secure).
7. You’ll be prompted to set a password. For maximum security, uncheck the option to remember it in your Keychain — otherwise, anyone logged into your Mac account could technically unlock it without knowing the password.
8. Under “Image Format,” choose “read/write disk image” so you can add and remove files freely afterward (a “sparse disk image” is also worth considering if you want the file size to grow dynamically rather than reserving the full space upfront).
9. Click “Save,” and Disk Utility will generate your encrypted .dmg file.
10. To use it, simply double-click the .dmg — you’ll be prompted for the password, and once entered, it mounts as a normal-looking drive in Finder where you can drag files in and out.
11. When you’re done, eject it from Finder’s sidebar (or drag it to the Trash icon, which becomes an eject icon). The moment it’s ejected, the contents become fully inaccessible without the password again.

This is a great lightweight alternative to VeraCrypt or commercial encryption tools, and because it’s entirely built into macOS, there’s no risk of a third-party app going unmaintained or being discontinued. You can also back the .dmg file up to cloud storage — the encryption travels with it, so even if someone accessed your cloud account, they still couldn’t open the file without your password.