See this for recent stuff. See this concerning Mac startup keys including Open Boot.

Acquire a virtual teletype (VT).

A 1990’s vintage terminal program on any personal computer will probably do. It must know how to set the interface rate of the UART of the computer.

Attach VT to serial port of target Mac.

I think that the Mac’s modem port is the one conventionally used by OpenFirmware. Macs use a small round DIN plug and I found a cable with two such. It did not require a …null modem” which might be required if you use the more standard (but rarer) RS 232 DB-25 pin plugs.

Guess the baud rate of the serial port on the target Mac.

One Mac 7200 talked at 38400. Perhaps that is its usual rate for OpenFirmware or perhaps it was merely the rate most recently set by the software for talking with an ISP.

Instruct VT to communicate at that rate.

With ZTerm 1.0.1 on the Mac, you can create an entry in the “Dial Directory” by selecting “Directory” under the Dial menu, then clicking New. It asks for a rate. You needn’t enter anything else in the window but the rate except, perhaps, a “Service Name” which will label your windows.

Boot the target Mac

Either power-on or “Restart” under the Special menu will do.

Very quickly (within a very few seconds) hold down the chord: Cmd-Opt-O-F

The ROM looks for this combination very quickly, even before playing the Mac chord.

At this point a greeting should appear on VT announcing Open Firmware. A Forth interpreter is talking to you and here is a trail of crumbs that I left behind as I got to the point where I could poke around.