Try this link:
http://www.chicandjo.com/2009/09/03/...le-cover-case/
It is a detailed tutorial.
Well, when I clicked on the above link, it took me to Chic and Jo's page but I didn't find the tutorial. I'll look some more!
I found it by doing a Google Search "How to make a custom kindle cover". The tutorial is about the 4 th item down on my screen. Hope this helps.