Creating a root filesystem with rootstrap

"One day, I needed a Debian Sarge root filesystem to be used with User Mode Linux, because the official website of UML only provided Debian Woody images. This short document explains how you can create your own root filesystem and run it through UML.

In the document, host is the computer on which you're working, and the target is the system we're trying to build."

http://thomas.enix.org/HowtoRootStrap