I recently upgraded my Samsung Nexus S to Cyanogenmod 10.1.2 as there won’t be factory images from google anymore. The phone has to be unlocked as described in previous posts in this category.
First I downloaded the following files:
Then I copied the two zip-file to the sdcard of the phone and bootet into the bootloader (Hold louder and on/off button to power on) to flash the recovery image from the connected notebook (you may need superuser rights on your local pc):
./fastboot flash recovery recovery-clockwork-188.8.131.52-crespo.img
After a reboot again into the bootloader I first made a factory reset, mounted the sdcard and flashed the two downloaded zip-archives beginning with cyanogenmod (the order is important). Then I rebootet my phone and it started nicely into my new cyanogenmod.
Notice! I’m not responsible if you brick your phone while following my instructions.