From Android Wiki
Added emphasis to the phase of moving the update, unzipped, to your phone, as I glossed over this detail and nearly bricked my phone.
Consider adding advice on possible snags in the install, if user forgets to copy the file over they can mount their SD card from console using this command
" To mount the sdcard in recovery type the following in adb
echo /dev/block/mmcblk0 > /sys/devices/platform/usb_mass_storage/lun0/file
thanks to JesusFreke for figuring this out. "