Nexus S downgrade with Clockwork Mod
You have to use fastboot.

That is all.

This is from my command line command history:

532  ./fastboot-mac flash bootloader bootloader-crespo4g-d720sprke1.img

533  ls

534  ./fastboot-mac reboot-bootloader

535  ./fastboot-mac flash radio radio-crespo4g-d720sprkh1.img

536  ./fastboot-mac reboot-bootloader

537  ./fastboot-mac flash recovery recovery.img

538  ./fastboot-mac flash boot boot.img

539  ./fastboot-mac flash system system.img

540  ./fastboot-mac flash userdata userdata.img

541  flashboot erase cache

542  ./fastboot-mac erase cache

543  ./fastboot-mac oem lock`