Make high sierra startup disk from Mojave using external SSD
Forgive me if this is posted somewhere or it's super-obvious but I just cannot find what I think that I am looking for.
I just got a new iMac (my last one died after 9 years) and it is running Mojave. I want to create a High Sierra Startup Disk on an external SSD so that I can try to revive my 2011 (Early) Macbook Pro to get some files. None of the startup recovery commands have worked on it. I can't seem to figure out how to do it. Basically, I want to be able to hopefully be able to access the Macbook Pro's HD one last time and my only thought after what I've already tried is to do the above.
Any suggestions? I some familiarity with Terminal.
iMac 27", macOS 10.14