Step 1: Head on over to the official ClockworkMod Google+ page and download the necessary .img file for your Galaxy Nexus or Nexus S.Step 2: Make sure to boot your device up into Fastboot mode. This is achieved by powering off your device, and then powering back on whilst holding the volume up and down buttons together.
Step 3: Flash ClockWorkMod (fastboot flash recovery the-name-of-your-img-file.img).
Step 4: Reboot the device into the operating system (fastboot reboot).
Step 5: Once the reboot is complete, you now have the ClockWorkMod recovery installed on a permanent basis.
