The object is to obtain a code for the car's key, then to cut a new one. It would seem that this is ordinarily done by a sort of bootstrap procedue. First the interior of the car is opened by slim-jimming the lock if possible. Mine was already open. Then, the trunk is opened from the interior, by pulling the lever next to the driver's seat. That is, unless the trunk is in lockout mode, which mine was.
If the side door is not openable by slim-jim or the trunk is unopenable by lever, the alternative is to make a key by impression. The locksmith wiggles a blank in the lock, leaving impressions where the tumbler pins are. Cuts are made in the blank with a rattail file, and the process is repeated; the pins stop leaving marks when the cuts are of proper height. The door or trunk can be opened with the new key, usually with a bit of wiggling as the fit isn't exact.
What follows is removal of a lock cylinder, on which a code is printed that the locksmith can then look up in a reference manual and use to cut a proper key. Some cars--such as, unfortunately, my 2000 Mitsubishi, require a third step: resetting a code in the computer. Many newer cars have a reprogrammable RFID tag in the key that needs to send the right number to the computer in order for the car to be started. No code, no spark. The locksmith has a device not unlike an engine scanner which can reprogram the computer to match the new key; old keys can be reprogrammed using a less portable device.
Interesting stuff. $300 worth of interesting? Maybe not. The Coso Petroglyphs--the reason I was there in the first place--are worth seeing and, as the American Altamira or Lascaux, less well known than they ought to be. Photos to be posted after everything is processed and scanned: so far only my Fuji Acros is back from the lab.