Can't seem to find that particular How To for some reason, but it is early and I'm not overly awake yet! But I do have a reasonable amount of experience in using most products short of a machine polisher, and I've found that while clay is great, it doesn't really help much with swirl marks - it's more of a preparation for the polishing stage. Happy to be proved wrong though, of course.
I'd love to pay for a proper detail, but it seems like they're £££
One day...