Flashing ClockworkMod 6 is a one tap job, provided you have unlocked bootloader and gained root access, already have ClockworkMod Recovery 5 flashed and ROM Manager installed on your Galaxy Nexus. All you need to do after meeting these prerequisites is launch ROM Manager and select the appropriate option from its main menu. That is all it takes.
The other, slightly more difficult method is to set up the Android SDK and flash the new recovery manually using the fastboot flash recovery <insert recovery .img filename> command.
As of typing, ClockworkMod 6 is available on all variants of the Galaxy Nexus (GSM, Verizon, Sprint). You can download the recovery image for your specific variant from the link below.
Download ClockworkMod 22.214.171.124 for Galaxy Nexus [Official Website]