Install Mac OS on external drive: disk locked

Of course after hours of trying, I found an answer soon after I posted.

1. I connected the external drive
2. From the app store, I found Big Sur and clicked "Get". Because I'd already downloaded the installer, it started after a few moments
3. I clicked continue and accepted the Ts & Cs
4. When the "select disk to install" came up, only the internal disk was shown so I clicked the "Show all disks" button
5. The external disk was also shown, so I selected it
6. A message said the disk couldn't be used as it wasn't APFS with GUID partition map
7. I backed out of the installer, ran disk utility, erased the disk and formatted it as APFS with GUID
8. Ran steps 1 to 4 again, this time the install proceeded
9. 20 minutes later I had a bootable external drive.

The above is far simpler than the MacWorld instructions.