You REALLY should read the XDA thread I linked...twice.. but if you havent by now, i guess you wont. Here is the section of that post containing the steps you are currently on. Written by Samcripp, all credit goes to his writting of this. This is an excerpt copy/paisted for those unable to view the origional thread:
4.a Now you can flash the update.- turn off the phone
- turn the phone back on holding volume down
- once fastboot appears on screen press down on volume and scroll until you see "Android Recovery" as an option
- press up on the volume
4.b The Android Recovery (NOT CWM!)- in this screen the options are hidden
- to make the options appear press on the right or left at the bottom of the screen (this make take a few tries)
- once you see the options, use the volume rocker to move to the second option
should read something like "apply update.zip from sd card" and press ok on the bottom left side of the screen
- ONCE STARTED, DO NOT INTERRUPT THE PROCESS OR YOU WILL BRICK YOUR PHONE.
5. Once the phone is updated, allow it to reboot, and enjoy 4.5.91 stock!