IMPORTANT!!!
The below set of steps worked fine for my device.
My device specs:
Google Nexus S (Not 4G), T-mobile, I9023,
had ICS 4.0.3 and downgraded to 2.3.6
I'm pretty sure the downgrade process is similar with other devices
(like Nexus S 4G) BUT with some differences, so I wouldn't attempt to downgrade.
The basic idea is that when downgrading, you have to use clockworkmod,
because the stock recovery won't let you downgrade, works only when upgrading.
Before you begin, make sure you've backed up all of your data and stuff,
because the process will wipe out all files from the internal and external storage.
So here we go:
1- get (google it) fastboot with the usb driver and clockworkmod
recovery image (I used 5.0.2.0), get clockworkmod from here:
http://download.clockworkmod.com/recoveries/recovery-clockwork-5.0.2.0-crespo.img as for fastboot, Google it and you should find a zip file with
+ fastboot.exe
+ AdbWinApi.dll
+ AdbWinUsbApi.dll
+ usb_driver folder in which contains the usb driver (needed only on windows)
2- get the proper ROM image for your device here (I got the 2.3.6 for I9023):
http://forum.xda-developers.com/showthread.php?t=1063664 Be careful to choose the right on for your device, and
get the FULL not UPGRADE
So now you should have fastboot, clockworkmod recovery image, and
the ROM image
3- Put in the same folder, anywhere on your disk, all the above files and
folders. In addition, copy a cmd.exe or just open one and change the
"working directory" (with cd command) to the folder in which you have
placed the files. You should have the below:
[Folder] usb_driver
[File] fastboot.exe
[File] AdbWinApi.dll
[File] AdbWinUsbApi.dll
[File] recovery-clockwork-5.0.2.0-crespo.img
[File] the_ROM_image.zip ( I got 1ada25375426.signed-soju-ota-189904.1ada2537.zip, that's the 2.3.6)
4- [IMPORTANT]
Before you turn off your device back up everything you need,
because all file will be erased, even those on your internal device.
After that turn off the device.
5- Hold volume up and power key to boot into bootloader
6- Connect your device via usb cable, windows should recognize the
device, then install the driver.
7- Either open a command prompt and "cd" to the directory in which your
files are located, or if you've copied a cmd.exe into your folder,
then open it, either way you have to run it as an administrator just in case.
8- Now, having your driver installed and the device still connected, type in:
fastboot devices
that should give the serial number of your device
9- Then type in:
fastboot oem unlock
go to your device and select YES (with volume button) and confirm
with power button. After confirmation, go to your PC,
it should display sth like (I typed the time on random):
...
OKAY [10.123 s]
finished. Total time: 10.123 s
10- Next you need to flash the clockworkmod recovery image, type in:
fastboot flash recovery <your_clockorkmod_image_file.img>
mine was "recovery-clockwork-5.0.2.0-crespo.img" so I typed:
fastboot flash recovery recovery-clockwork-5.0.2.0-crespo.img
11- Next, on your device's bootloader menu, navigate using the volume
keys (up and down) and select Recovery and hit the power button to confirm.
Now your device should boot into recovery (the clockworkmod recovery)
12- From the recovery menu select "mounts and storage" (again navigate
with volume buttons and select with power button), then select
"mount USB storage" and then go to your computer on which
a removable disk should pop up
13- copy your ROM.zip file from you disk over to the removable disk storage,
when finished go back to your device and hit "*****Go Back*****" and again
"*****Go Back*****" and now you should be on clockworkmod recovery
menu, from which, select the "install zip from sdcard", then select
"choose zip from sdcard", and select the your ROM.zip file you've just
copied over, then select "Yes - install your_ROM_file.zip
mine was "Yes - Install 1ada25375426.signed-soju-ota-189904.1ada2537.zip"
Now, wait until the installation is finished (it might take a couple of minutes)
When finished hit "*****Go Back*****" and then "reboot system now"
Now the device should reboot with your new android OS.
14- Now you need to lock the bootloader back on, so power off your device,
press volume up and the power button to boot into bootloader,
connect your device via usb, and in the same cmd type in:
fastboot oem lock
and you'll notice on your device is changed back to "Lock State - Locked".
Reboot...
And that is it.
Now, if you prefer to do a factory reset you can do so
from the settings menu or from bootloader, but is not necessary