The quickest way to skin the default theme (only using CSS) is as follows:
1. Copy /magento/skin/frontend/default/default folder to /magento/skin/frontend/default/newtheme
2. Go into your admin and turn off caching.
3. Go into your admin and tell Magento to use your theme.
This is done under System -> Configuration -> Design (I think!? Doing it from memory at the moment).
4. Get firefox with the firebug extension so you can see which CSS controls which parts of the page.
5. Begin editing. The three files you will mostly be editing are boxes.css, reset.css and menu.css in /magento/skin/frontend/default/newtheme.
Bear in mind that by only editing the CSS you can’t change the position of blocks on the page, only their styling.
Hope that helps.