How to make a bootable external High Sierra drive?
I am running Mojave on a MacBook Air 2017 and I need to create a bootable version of High Sierra for one unsupported program on an external USB drive. I followed a link on the App Store, but it only gave an update, not the full install package. I tried getting an installer for Sierra, thinking I could install it on the USB drive and then upgrade, but when I ran it, the USB drive didn't even show up as an option. I tried creating a new volume on my hard drive thinking I could maybe do it from there, but got the same result. What is the best way to do this?
MacBook Air 13″, macOS 10.13