The Drupal.org site has a great deal of information on how to theme for Drupal 6. Here is an overview with links to the longer articles.
If you have never themed Drupal before, you are probably wondering about what is involved in the process. Theming for Drupal requires an understanding of:
- Proper front-end web development: (x)HTML + CSS
- The Drupal terminology / theme content tags
- PHP knowledge is a plus, but not necessary
This is a basic video demonstrating the new Drag And Drop capabilities of the Drupal 6 interface. Its advantage becomes more apparent on the Blocks admin page that can get cluttered with larger sites.