I found a lot of instructions on the internet about how to do this but many were incomplete or were simply just wrong.
The only way I got it to work was to add these lines to config.txt on the existing SD Card :-
program_usb_boot_mode=1
program_usb_boot_timeout=1
Then reboot. This tells the Pi to check for other boot devices.
Once done, in terminal type vcgencmd otp_dump | grep 17:
The output should say 17:3020000a where the 3 indicates that USB boot is enabled.
Now plug the SSD into a computer and using the Raspberry Pi Imager create a new image on it.
Plug the SSD in and remove the SD card. Reboot. It should now work.
I tried to use the SD card copier to copy the SD card image onto the SSD and it didn't work. I kept getting 'Kernel panic-not syncing: VFS: unable to mount root fs on unknown-block(179,2)'
Apparently if you have a Pi 3 (not the B) then you need to format a SD card and just put the file bootcode.bin onto it (available from Pi site). Then put this card in the SD card slot. This tells the Pi to boot from the SSD.
Norman
