From PaulGuWiki

• Go to your MediaWiki skins directory.
• Copy the monobook directory and name it with the name of your skin, for example: mywikiskin.
• Copy the MonoBook.php file and name it with the name of your skin, for example: MyWikiSkin.php.
• Edit MyWikiSkin.php and change the class name to SkinMyWikiSkin, for example:

class SkinMyWikiSkin extends SkinTemplate.

• In MyWikiSkin.php, change make the other class to have your skin name together with the word Template, for example:

class MyWikiSkinTemplate extends QuickTemplate.

• In MyWikiSkin.php, make $this->skinname to have a string value of your skin name, for example: 'mywikiskin'.
• In MyWikiSkin.php, make $this->stylename to have a string value of your skin name, for example: 'mywikiskin'.
• In MyWikiSkin.php, make $this->template to have a string value of your skin name properly capitalised with the word Template, for example: 'MyWikiSkinTemplate'.
• In your mywikiskin directory edit the image and CSS files.
• Edit and customise MyWikiSkin.php as you please.
• In LocalSettings.php set $wgDefaultSkin = 'mywikiskin'.