This installation worked in Ubuntu 10.4 but fails in Ubuntu 10.10 and 11.4.
Webmin installs as an application under Ubuntu the same way you add any other program. The one difference is the adding of the Webmin repository to the Ubuntu Software Centre so that Ubuntu knows where to look for Webmin.
Log in to your wonderful Ubuntu machine. Change the desktop background to something distinctive so you are reminded that this is The Machine!
Select Applications then Ubuntu Software Centre. In Ubuntu Software Centre, select Edit then Software sources then Add. Fill in the following information.
deb http://download.webmin.com/download/repository sarge contrib [Add source]
Back at Ubuntu Software Centre, search for Webmin using the search box in the top right corner.
Select the Webmin line returned from the search. (It may take a few seconds to find Webmin.)
You will have to type your password into the Authenticate box to proceed.
You will get a message about installing untrusted software. Select [OK]. The installation will stop without a message.
Go to www.webmin.com/deb.html. You have to download and import the GPG key mentioned on the Debian page. I downloaded the key file then used Edit, Software sources, Authentication, Import Key file. The key file did not take immediately. I had to exit Ubuntu Software Centre then go back in and wait for some sort of update. I searched for Webmin then select install several times several seconds apart. The install started about the third time and a little progress bar opened up.
When the installation is finished, you will see a [More info] button providing login details. You can now login to Webmin.