Install/Flash CyanogenMod 9 Nightly On Galaxy Nexus/Nexus S/XOOM

First you must root your device and flash the custom ClockworkMod Recovery to be able to flash this Rom.
Step 1: Download the CyanogenMod 9 ROM .zip file for your device from the link below:

• Galaxy Nexus GSM/CDMA
• Nexus S GSM/CDMA
• Motorola XOOM WiFi/LTE

As of writing, these are the only devices that have received new CM9 nightlies. More nightlies will be out for other devices which you can download from here [CyanogenMod Website].
Step 2: Place the .zip file directly under /sdcard  on your device’s internal storage.
Step 3: Shut down your device. Once it is completely turned off, you must press and hold a specific combination of buttons to boot into ClockworkMod Recovery. This combination is different for different devices so you’ll have to search for it on Google.
Step 4: Take a NANDroid backup.
Step 5: From the main menu, you must wipe data/factory reset. Similarly, you must wipe cache and wipe Dalvik cache from advanced menu.
WARNING: This will delete all personal data on your device.
Step 6: And now, for the most important step. Navigate to install zip from sdcard > choose zip from sdcard and choose the .zip file you transferred to /sdcard in Step 2. This will flash CyanogenMod 9 Nightly to your device.
Step 7: From main menu, choose reboot system now.