If you haven't already, plug in your external drive, or join to the network your NAS is connected to.

Click the Select Backup Disk button.

Select the disk you'd like to make backups to from the list.

External backup drives must be HFS (Mac OS X Journaled) formatted to work with Time Machine, not APFS, NTFS, etc.

Networked devices should appear automatically on this list. Consult your NAS' manual for assistance.

Check the Encrypt backups checkbox for additional data security, especially if your backup device is portable.