Simply:

1. Copy the URL to the latest Debian version at http://webmin.com/download.html
2. Run "wget [URL]" from your command line, replacing "[URL]" with the URL you got from the Webmin site (e.g., http://prdownloads.sourceforge.net/webadmin/webmin_1.460_all.deb).
3. run "dpkg -i [the file you just downloaded]"
4. If you get an error saying that webmin depends on other modules not yet installed, install them: sudo apt-get install libnet-ssleay-perl libauthen-pam-perl libio-pty-perl libmd5-perl (include any others that were listed as well)
5. Webmin will then automatically install itself because it was waiting for you to do what you did in the previous step - pretty smart huh?
6. If for some reason webmin did not auto-install, just run "dpkg -i [file you downloaded]" again.