you need unlocked bootloader.
on the beging have device conected to computer by USB
step 1 (on PC) 1.1 copy update-XXX-XXX-XXX.zip and GAPPS ADDON to sdcard
1.2 extract from update-XXX-XXX-XXX.zip the file called kernel.elf
step 2(on device) 2.1 - power off the device
2.2 - unplug usb cable
2.3 - put device in fastboot mode (volume up + power) - blue light on device
step 3 (on PC)
3.1 - flash new kernel "fastboot flash boot kernel.elf"
3.2 - reboot device "fastboot reboot"
step 4(on device) 4.1 - when FreeXperia Logo appears and amber light will be lit press several times volume key to enter recovery
4.2 - in recovery choose "INSTALL ZIP FROM SDCARD" and select update-XXX-XXX-XXX.zip (rom)
4.3 - in recovery choose "INSTALL ZIP FROM SDCARD" and select gapps-xxx-xxx.zip (gapps addon)
4.4 - wipe device (this is needed only if you come from another ROM, if you are upgrading an CM9 release this is not needed unless is specified)
4.5 - reboot device
ENJOY AN UNOFFICIAL CM9 RELEASE BROUGHT TO YOU BY FreeXperia Team
cm9.0 GAPPS ADDON - http://cmw.22aaf3.com/gapps/gapps-ic...317-signed.zip
Face Unlock ADDON - http://cmw.22aaf3.com/gapps/gapps-ic...131-signed.zip