When you have the bootable installer complete - Launch OCLP app. Click Build and Install OpenCore to that external drive. If you have multiple volumes on that external, OCLP will show only the external device, and not each individual volume that you might have. If you have only one external drive, just choose that one. (if you have multiple external storage devices, you want to make sure that you add OCLP to only that drive, the one with the partition that you want to use for a bootable installer.
And, then you need to remember to hold Option when you boot to that installer, choosing the EFI partition first, then press return to load that EFI, and then choose your installer partition to boot to your installer.
It will be simple, after you do that a time or two.
Back with an update:
This was a terrific help, except for one main stumbling block, which I think I overcame mainly by chance.
When I began the install, was greeted by the now 2 Install macOS Ventura, with no idea which one was the newly created one for the Mini. Basically a crapshoot, but since the new one had appeared at the right on the Desktop, I decided to select the one at the right on the first reboot, and it turned out it must have been the correct one, since I was able to finish the installation. Had I been wrong, after watching innumerable progress bars and screen changes, it would have resulted in some kind of message to the effect that the installation was configured for the wrong drive--and I would have had to start all over again. OUCH!
After that, it was basically looking at innumerable screen changes/progress bars, which, unlike on any previous attempts, I decided not to second-guess at all, and just allowed whatever it wanted to keep going, and going, and going, for hours (compared to the iMac, the Mini is quite slow).
Also, and this might or might not have been a big one, I had sent over the Ventura Installer from the iMac to the Mini, which resulted in making me, not System, the owner. Hadn't noticed that before. So, not trusting that installer, decided to get a brand new installer directly at the Mini this time with the correct permissions - which I realize I could have changed without getting a new installer, but decided to start fresh.
Also, no matter what I tried, I was always brought to the Recovery page, and from there had to select the Ventura installer to proceed-don't think that's mentioned anywhere in the OpenCore directions. Could be that I was brought to Recovery because I installed to a partition on the Internal drive, which means cohabitation between High Sierra and Ventura. I realize that this is not recommended, and I won't go into all the details at the moment, but based on my experience at the iMac, this in fact works out best for me. What I like about this is that, one way or another, I can always select an item I need from the HS volume and bring it over to the Ventura. Just too much at the HS to allow a complete migration, and don't have the patience to select among hundreds of items on a first migration. (So basically did the migration from the Ventura CCC backup at the iMac - which also has its problems, since it brings over some bugs and incompatibilities, which I'm presently trying to sort out).
Thanks again for all the help with this.