These are the steps I took to root and have CWM installed on the Galaxy Nexus based on mskip on forum XDA :
PreRequistes on PC: Have the ADB and Fastboot installed
1. INSTALLING ADB/FASTBOOT DRIVERS
The first thing you need to do is to install the adb/fastboot drivers. These are needed so that you can unlock your bootloader and flash CWM (ClockworkMod) Recovery to your phone.
a. The easiest ones to use are PdaNet for Android, download and run the 32-bit Windows 7/Vista/XP or 64-bit Windows 7/Vista and follow the instructions to install (selecting Samsung as your device manufacturer if asked).
b. After the drivers have been installed disconnect the usb cable and shut down your phone. Keep holding the ‘Volume Up’ and ‘Volume Down’ buttons and press the ‘Power’ button to boot the phone into FastBoot Mode.
c. Go to your Control Panel on your PC and open the Device Manager. You want this open so you can see the FastBoot driver is installed or so you can install it manually.
d. Connect the usb cable. In the Device Manager a new item called ‘Android 1.0’ should pop up and then the driver should install automatically. Once it is installed you will have a new section in the Device Manager called Android Phone without an “!” meaning its installed correctly.
Note 1: If the driver doesn’t install automatically then double click on the Android 1.0 item, then click on the driver tab and select ‘update driver’. Select ‘browse my computer’ and then ‘Let me pick from a list’. If no items appear in the list then untick ‘Show compatible hardware’ and find the Android adb driver or Samsung adb driver.
2. USING MSKIPS’ GALAXY NEXUS TOOLKIT V3.1
a. Download the Galaxy Nexus Toolkit file and run to self extract the files to your PC. The default extraction folder is C:\Galaxy Nexus ToolKit
b. Once the files are extracted put your phone into Fastboot Mode (Hold ‘Volume Up’ and ‘Volume Down’ buttons whilst pressing the ‘Power’ Button) and run the RUNME.bat file (DO NOT RUN IN ADMINISTRATOR MODE OR IT WONT WORK) from the installation folder on your PC. With V3.0 you can also connect your phone whilst it is in Android as you can boot to Fastboot from the MainMenu.
c. The first screen gives you information about setting up adb/fastboot drivers if you havent done already.
d. The second screen gives information on the various options available from the Main Menu.
e. The next screen will ask you which model and Build you have (GSM or LTE) and set up the Main Menu accordingly.
f. When you enter the Main Menu you will see ‘Serial Number’ at the top. If you have installed your drivers correctly then you will see your phones serial number under Fastboot Mode or Adb Mode depending on your connection. If you see this then your device is connected and you can carry on.
g. Each option has full information on what it is and how to use it.
h. After you have finished you can either boot straight into CWM Recovery to flash a Custom Rom (if you have already downloaded one and copied it to the root of your Internal Memory on your phone) or boot back into Android as normal.