Run Sim390[1] with MUSIC/SP. Then get a TN3270 virtual terminal software to connect to it.
3270 is screen-based--think of it like filling out web forms and submitting, then receiving the results. This is far different than line-based terminals (e.g. your typical *nix TTY).
Compared to IBM's terminals, DEC's terminals seem completely unremarkable.
It would have much easier if the applications were TTY line-based.
It was incomplete, as anything offering a WinForms GUI over a 3270 client would necessarily be, but still very useful. A 3270 terminal emulator was also available; most folks quickly learned to use it for tasks the GUI didn't support, and some of us ended up preferring it for some tasks the GUI did support since it tended to be much more accommodating of muscle memory.
In retrospect, it must've been a real engineering challenge to integrate with a 3270 client whether via screen-scraping or the wire protocol. Like HTML, but with structural information only designed to be parsed by the terminal, or in the screen-scraping case with no structural information at all.