The first version of the HOWTO can be found on http://scorpius.homelinux.org/~marc/debian-squeeze-on-sheeva-nand.html
. I've uploaded all the images to flash the SheevaPlug from scratch.
Most of the text is about creating the images, if you're not interested in the process, just browse down to the end (almost) of the page. The final images are available from the web page and should make flashing it pretty trivial.
If something is not quite clear, just let me know.