Hmm.. I thought you would remember some of this from rooting. I'll try to make it more clear.
Fastboot commands are commands that you send to your phone through command prompt. There are also adb commands. There are two ways to put the phone into fastboot usb mode. One is to connect to the pc, and enter
adb reboot bootloader into the command window. That will reboot the phone into fastboot and since it's plugged in should say fastboot usb in red letters on the phone display.
The other way is to power off. Hold volume down+power to get to your bootloader. From there, select fastboot. Once you plug the phone in, you should see fastboot usb in red letters.
This means that you can now send fastboot commands. Just like when you rooted. Remember
fastboot oem get identifier token and
fastboot flash unlocktoken unlock_code.bin and
fastboot flash recovery recovery.img etc. etc....
So what you need to do is first put the phone in fastboot usb mode per one of the methods above. Then you will use the fastboot command
fastboot oem lock
That will re-lock your bootloader. From here your phone should still be in fastboot usb mode. Run the RUU.exe on the pc and follow the instructions.
It's possible you need to lower the mainversion of your phone before the ruu will let you run it. So if the RUU returns an error, put the whole setmainversionlow.zip anywhere on your sd card. Then boot into your recovery.
Do this by powering off the phone again and booting into the bootloader. Then select recovery. From there install the zip file just like you would with a rom. Just choose install from sd card then browse to the setmainversionlow.zip and select it. It will only take a couple seconds to flash.
Then get your phone back into fastboot usb mode again and attempt to run the RUU again.
I hope that makes it more clear??
