I don't know of a specific guide, but I would guess that having Steam install the games directly to the drive would be the simplest way of doing so. If the person doing this for you already has some of the games installed for their own Steam account, I believe they should be able to also copy from their current steamapps folder to the external drive's steamapps folder.
Just warn the person doing this for you that (depending on the size of your library) it might take a few days (or more depending on their bandwidth) to download everything. I installed my entire library at once (around 500 games at almost 2 TB) a few weeks ago and it took several days to download everything, and I have the max ATT Uverse internet speed in our area.
To expand on this idea, for any games you buy after receiving your HDD, you could again have someone download the new games for you but this time transfer those new games' steamapps folder to a flash drive that they send to you. You then copy the steamapps folder to your HDD and send the flash drive back. You could keep your library pretty much up to date this way.
I think Visualante2 was referring to the backup tool itself failing to work properly.
Edit: Yep.