Cynaogen Mod 7, a full working release is available for Nexus S running Gingerbread 2.3, Android’s latest platform released earlier in December 2010. Developers and testers have reported successful deployment of this mod on Nexus S and hence, is available for users. Requirements include rooted Nexus S, and custom ROM. Copy these files after downloading them to your Android device. Download ROM Manager and select ‘Flash ClockwordMod Recovery’. Flash ROM from SD card (ROM Manager) and from the concluding options, navigate to CM7 zip file and install. Wait till the phone boots into recovery.
And you have successfully installed a custom ROM on your Google device.
Download CyanogenMod 7 for 2.3 Gingerbread Custom ROM