1. Unlocking the Bootloader

The bootloader of the Nexus S is locked initially. You'll have to unlock it using a tool called fastboot. Warning: Since unlocking the bootloader and rooting the phones gives access to all the private data, by circumventing the Android's security model based on Unix, all your phone's data and data on the internal SD card will be erased for security reasons. Hence take a backup of the important data into your computer before you proceed.


You should have Android SDK installed in your machine. For this tutorial, I use a mac. Post Android SDK installation, download and place fastboot (which is just a single binary file) in your path or if you're comfortable, run it from any directory, using a terminal.

I would prefer you to place fastboot in the 'tools' directory of your android SDK installation.

Step 1. Reboot your phone into the bootloader by shutting it down and then turning it on by holding the volume up button + power button and you should see this.

Step 2. When on the bootloader screen, connect your phone to PC/Mac and run the command

fastboot devices
which should give an output similar to

List of devices attached 
HTXXXXXXXXXX	device

Step 3. Once, it's confirmed that your device is recognizable by fastboot, run the following command to unlock the bootloader. 

fastboot oem unlock

Read the warning on the phone and accept it.  That's it! Now you have an unlocked phone or bootloader, perhaps! :D This can be confirmed by seeing noting the last line on the first paragraph of the bootloader screen that reads

LOCK STATE - UNLOCKED

Next you shall run  

fastboot reboot

reboot your phone. Another hint to know that your bootloader is unlocked is to reboot your phone into the os and you can notice that beneath the Google logo appearing while it boots, there should appear an unlocked-lock icon!

It should be no surprise that you can run

fastboot oem lock

to lock the bootloader if you wish!