So if you made a nandroid backup of your pure stock system, before you rooted and flashed anything custom, you're pretty well off and don't need to mess with any of the steps above.
First off, you'll need to be able to get into recovery. If you can't, then you can forget this and go through post 1 above.
Once you're in recovery, simply restore the nandroid of your stock system. Once that's done, all you need to take care of is recovery, which is potentially extremely simple.
Recovery option #1 - there is a file in the /system directory called recovery-from-boot.p - if that file is untouched, every time you boot up, it will ensure that you're recovery is the stock .img, and not a custom one. To test this, reboot after restoring your stock nandroid, then power down and get back into the bootloader, then into recovery again. If a green android with a red exclamation pops up, you're good to go.
Recovery option #2 - download this file (mirror 1) (mirror 2), put it in the same directory as your adb/fastboot tools, reboot into the bootloader, and in the command prompt input fastboot flash recovery recovery.img - that's the stock recovery.img from the stock system .zip in the first post.
Most likely option #1 will work, but if you need to manually flash the stock recovery for any reason, option #2 is it.
Lastly, if you want to re-lock the bootloader, simply get into the bootloader, and in the command prompt input fastboot oem lock - and you're done.