Convert APFS to Mac os extended on external SSD.

Hi,I am wanting to create a startup disc on an external SSD with High Sierra on it, so that i can run apps that are not compatible with Catalina.

I don't want to revert my main disc back to High Sierra as that would mean erasing after back up of the disc, I do not have Time Machine backups going back to pre OS Catalina days.

I need to format the SSD to Mac OS extended (Journaled) so that I can then install High sierra onto it.

Does anyone out there know how I can do this please?

Cheers

Eric