Because modifying an initial ramdisk is something I had to do every once a while and I am pretty oblivious when it comes to “exotic” commands, I will persist the knowledge at this point.
First of all, modern linux kernels (like everything above 2.6 or so) use an initrd that is a gzipped cpio-image (use file to be sure).
It might be possible to simplify some steps of this howto, but I tend to clarify things to avoid confusion.
I stumbled over the newc format of cpio. Actually, I think that there’s not pretty much of a choice here.
The documentation doesn’t say much about that, just use it.