Since it took some time to me to acheive the same thing, I will tell you what to do :
Yes you'll have to create your own ram disk in order to let the kernel load before the usb support in order then to be able to mount your root filesystem on the usb drive. Else it will try to access your usb drive to find the modules (instead of in the ramdisk you will do) so as to mount your filesystem, and here you see the problem.
The good thing is that you can create the appropriate ramdisk image by following step by step the instructions given there, no matter if you understand or not what you're doing :
Instead of using the rescue CD you can load a live cd as Knoppix for instance.
It worked for me, with FC2, so good luck !