I downloaded the sets (such as base.tgz) and the loader
from the net, using ANT internet suite.

I then formatted my 4.3GB IDE disc to RiscOS, but only on
the 300 first sectors.

Then I ran the program bb_riscbsd (from the loader) to make
the rest of it accessible to NetBSD.

Then I uncompressed the kernels with gunzip, before I 
downloaded them. You must set their datatype to "data",
since this is what the bootloader unnecessarily demands.

Then I started the bootloader, put the RISCPCINST kernel into it,
and started.

I formatted 2 partitions. 1 root partition, from track 300 onwards,
and 1 swap partition.

Then I typed "reboot" to get back to riscos, and used unixfs to
put the sets to the NetBSD disk, on /tmp.

Then I rebooted NetBSD, and mounted the disc with
  mount /dev/wd0a /mnt
  cd /mnt
and extracted the sets with the command:
  tar xvzpf tmp/base.tgz    a.s.o.
(Note absence of /, and presence of p and z in tar options.)
(The "inst" program did not see the *.tgz files.)

Then I booted it with kernel RISCPC and /dev/wd0a

It appears to work fine, except I get the message:
  /etc/rc.conf is not configured. Multiuser boot aborted.

The "INSTALL" guide was not of much use.