From PaulGuWiki

In order to use the skin GuMax for your own Wiki, you need to do the following steps:

• Download one of the GuMax skins from Mediawiki Skins
• Extract it into to desktop, and you will find a folder called skins
• Use a ftp client and upload all the files in the folder skins to the your wiki server folder skins. For ftp client, you can go to FileZilla and download; FileZilla is a fast and reliable FTP client that I'm using.
• Open your local configuration file LocalSettings.php, and then find:

$wgDefaultSkin = 'monobook';

change it to:

$wgDefaultSkin = 'gumax';

• Go to your web site and test the new skin. For registered users, they need to change the skin from My Preference.