First of all, how do you know this job isn't a good fit if you have only been there a week?
I would make sure that this new job isn't for you (this step is up to you to determine).
One more thing before you quit; if you haven't thought about it, maybe you could try telling your boss that you don't have enough work or that you feel you don't have any upgrade potential and see what he says. It's just another avenue to consider but it sound like you are pretty much set on going back.
After you make your final decision that this "new job" isn't for you, tell your (new/current) boss the following (feel free to restructure and so on):
"Hello (Boss' name), I need to tell you that I'm going back to my supervising job. I know I've been all over the place recently, but I'm trying to find the best fit for myself. I apologize for any difficulty I might have caused and thank you for your time and this opportunity."
I'm sure you were going to say something along these lines but I was just reinforcing that this is all you can really do. Yes, you seem a bit discombobulated and unsure of where you need to be, but it's not that big of a deal. Just tell him/her and move on. Your current boss more than likely won't think much of it.
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