The required packages are the following:
The kernel-patch-uml package is only necessary if you want to compile you self a kernel. This case isn't described in this document.
Using existing images don't require a kernel compilation, because the kernel to boot is already present in the image self.
To make an installation under Debian execute
apt-get install user-mode-linux-KERNEL_VERSION-lum-l
where KERNEL_VERSION is the kernel version that the Debian community currently provides.
This step isn't a really installation and you don't need to be root, therefore is recommend that you work as a normal user.
To boot a virtual machine you need a root filesystem. There are normally compressed images and no any Debian packages provides them. You must download your preferred image from the official homepage
http://user-mode-linux.sourceforge.net/dl-sf.htmlIf you want to work with a Debian image (this means that the virtual machine will be a Debian based system and not that you must use this if you are working on a Debian system) the last currently image is
Debian-3.0r0.ext2.bz2
After downloaded it you can extract it with
bzip2 -d Debian-3.0r0.ext2.bz2
The result is the following file
Debian-3.0r0.ext2
that is your root filesystem.
You can place this image anywhere you want, but please not in /tmp because with the next boot (of your pc) this directory will be completely erased.