How to use a serial console on Ultra 1?

[John Zbesko <johnzbesko@alum.mit.edu>]

Ah, the joys of working with old equipment. I recently re-booted my
Ultra 1 and the monitor/graphics stopped working. The machine sounds
like it's booting; the keyboard lights up; but the sun monitor goes into
a "no signal" state. Digging around, I found an old cable that connects
a com port of the back of Ultra with a (smaller) com port on a
laptop-in-docking-station running kubuntu.

How do I use a serial console and test if my Ultra is indeed working?