Tutorial

The first step you need to take if youre going to use a custom ROM on a Nexus device is to unlock the bootloader. All bootloaders -- even on a Nexus phone -- come locked. The big difference is that a Nexus phones bootloader can be easily unlocked with just a few commands.
Heres how to unlock the Galaxy Nexus bootloader. Note: This will erase everything on your Galaxy Nexus, so if you think its something youll ever want to do, we suggest doing it when you first get the phone.

1. Youll need to download and install the Android SDK.
2. If youre using Windows or Mac, youll need the Fastboot tool. (Download for Windows, download for Mac.) If youre using Linux, you can make your own Fastboot tool. (And if youre using Linux, chances are you already know how to do all this anyway.) Stick it in your sdk tools or platform-tools folder.
3. Reboot the Galaxy Nexus into the bootloader/fastboot mode. The easiest way to do that is to turn the phone off. Now hold the volume-up and volume-down buttons, and then press the power button.
4. Youll now see a cool looking Android roboto with his front panel open. Check to see that the text below it starts with "FASTBOOT MODE" in red. Now plug the Galaxy Nexus into your computer with the micro USB cable. Let any drivers install.
5. Now open a command line. type fastboot oem unlock. This will start the process and give you the screen you see above. Choose yes using the volume button and then press the power button. Your bootloader is now unlocked.

So now what? Now you can install rooted system images. Or entire custom ROMs. And a custom recovery. All sorts of stuff. Welcome to the real world.