This will install Ubuntu 9.04. If you want to install Intrepid (8.10), change "jaunty" in the above URL to "intrepid." I've found Jaunty to be stable for what we are doing as of this writing, and is set to release on the 23rd of April.
Once you've downloaded the ISO, burn it to a CD-R. For the mini.iso install method, your server will have to have an active network connection during the install. For most machines this should pose little problems, however if your network interface is not supported in the installer, this install will not work. You will need the Alternate Installer, which the steps should be similar. The benefit to using the mini.iso is that JUST the packages you need are downloaded, as opposed to the entire Ubuntu Desktop system.
Insert the new CD-R, and boot the machine you are going to run the install on. When the CD's "boot:" prompt comes up, put in:
and hit enter, following the installation steps. They are all fairly straightforward. When you are done, you will not have a display manager login, like most Ubuntu installs do, you will have a console login. To keep Ubuntu lean, and mean, all non-essential services are deactivated, that includes not using GDM. There is a way to start your GUI without a display manager (gdm), and we will get into that next.