...but seriously, folks
Here is Pokey's guide to NeXT greyscale monitor interfacing. Please leave your mind in the gutter. Thank you.
The 17" greyscale monitor connects through a 19-pin DB connector, same as the old Apple floppy drives. The cable provides power, so if you build your own cable, be sure to use a resonable gauge of wire. The pinout is straight-through one to one. Be sure to use a shielded cable, as you're trying to pass high frequency signals across this thing. Lastly, you can boot a NeXT headless. There is no 'fallback to serial' like most workstations have. Instead, find someone who has a monitor and keyboard, get into the ROM, (hold command-~ right after power-up) and hit 'p' for parameters. Say 'N' to 'Wait for keypress' and 'Sound out tests' but 'Y' to 'Port A As An Alternate Console'. Then, plug a terminal into port A and pull pin 6 of the monitor connector to chassis ground through a 470 ohm resistor to turn on the power. Do not short it directly; you may blow up a very rare computer.