To run Openstep 4.2 for 68k NeXT:

Start Previous, in settings:

In System, for Machine Type, select "NeXTstation" or "NeXTstation color". 
(Note: Color emulation gives less RAM, and Turbo is not implemented)

In Boot Options, select "SCSI disk" and check "Verbose test mode"

In SCSI Disks, set OS42_2GB.dd as "SCSI Disk 0"

Then click OK to start Nextstep.

During boot, Openstep will wait for a network configuration server and fail.
(Ethernet is not implemented yet) You must press CTRL-C to continue booting.

This VM will send you through the Welcome process at first boot.

To properly shutdown Openstep, press "F10".

For more information on Previous, see the following URLs:
http://www.nextcomputers.org/forums/viewtopic.php?t=2642
http://previous.alternative-system.com/

This version of Previous may occasionally hang at "root on sd0".
If that happens, just try restarting Previous. 
