Currently have Mojave OS, but want to add an APFS volume to also install High Sierra to have a dual boot

Hi guys,

I currently have Mojave OS, but my eGPU with nVidia only is CUDA compatible with High Sierra. I don't want to have to completely erase my current volume and rely on a backup drive, so I want to just set up another volume with an OS, but it is an older version.

What is this best way about doing this? I have already cleared up 75GB of space, and have already created the AFPS volume named High Sierra.