Before you start you should familiarize yourself with the boot PROM
of your machine.  The older Decstation 2100 and 3100 cannot select
a kernel from the command line. You need to set the bootpath
environment variable to point to the disk and kernel you intend to boot.

You should also examine the guide on the OpenBSD/pmax web site, which
will hopefully soon have more complete and more up-to-date
instructions than are given in the install document.

If you're installing OpenBSD/pmax for the first time it's a very good idea
to look at the partition sizes of disk you intend installing OpenBSD on.
Changing the size of partitions after you've installed is difficult.
If you do not have a spare bootable disk, it may be simpler to  re-install
OpenBSD again from scratch.

But if you don't have a second disk or plan to do an installation via
netbooting you don't have any choice about the partition sizes (at
least not for the root and the swap partitions) because they are set
in the simpleroot image to to 32M for root and 64M for swap. About the
rest of your disk you can still decide yourself.

Asumming a classic partition scheme with root (`/') and /usr filesystems,
a comfortable size for the OpenBSD root filesystem partition is about 32M;
a good initial size for the swap partition is twice the amount of physical
memory in your machine (though, unlike Ultrix, there are no restrictions on
the size of the swap partition that would render part of your memory
unusable). A full binary installation, without X11 or other additional
software, takes about 130MB in `/usr'.