Simple question: how do I create a High Sierra VM on macOS Catalina with Parallels 16?

I've been trying to find instructions, but the search function on the knowledgeable only gets me loads of useless hits (mostly about Parallels running on High Sierra).

So, I am looking for instructions that tell me:
- How to download High Sierra from Apple
- How to use that download to create a High Sierra VM

Thanks,

G