Don't know if any of this will be of help or not...
Here is the format spec...
http://www.openebook.org/2007/opf/OP...inal_spec.html
Here is the packaging spec...
http://www.openebook.org/2007/ops/OP...inal_spec.html
Here's a write up done on Teleread that might have some useful info...
http://www.teleread.org/2007/10/31/e...epub-creation/
Also the Mobil Read Wiki has some good info...
https://wiki.mobileread.com/wiki/EPUB
I know it's not really the step-by-step guide you asked for, but I don't think I've seen one.