The easy way to accomplish this is to just launch a NeXTstep box in your browser from https://infinitemac.org/

I don’t know if they have Improv pre-installed, but it will let you mount disk images from your computer.

(Personally I find the easy way too easy, so I have NeXT^WOpenSTEP installed on bare metal on a 5x86 box. But that’s me)