- download soju
- follow this guide
upgrade Android version
- download Cyanogenmod
I would start with a stable version e.g. something like this: .
- Put the .zip file with a USB cable to the Nexus-S root
cp update-cm-184.108.40.206-NS-signed.zip /media/8495-0F02/
- We also want Google Apps 
put the .zip file on the root folder
cp gapps-gb-20110828-signed.zip /media/8495-0F02/
- eject the USB drive from your PC
- start ROM manager (If you don't already have it get it from Android market)
- Flash ClockworkMod Recovery
- Reboot into Recovery
- Once the device boots into the ClockworkMod Recovery, use the side volume buttons to move around, and either the power button or the trackball to select.
Optional: Select backup and restore to create a backup of current installation on the Nexus S.
- Select the option to Wipe data/factory reset.
- Then select the option to Wipe cache partition.
- Select Install zip from sdcard.
- Select Choose zip from sdcard.
- Select the CyanogenMod update-cm-220.127.116.11-NS-signed.zip.
Optional: Install the Google Apps by performing steps 7 - 9 again and choosing the Google Apps gapps-gb-20110828-signed.zip.
- Once the installation has finished, select +++++Go Back+++++ to get back to the main menu, and select the Reboot system now option. The Nexus S should now boot into CyanogenMod.