There are actually only two files that you need to get a basic theme up and running: index.php and style.css. If you add these to a new theme folder (wp-content/themes/new-theme-dir/) then you can choose your theme from the dashboard (Appearance->Themes):
To populate this data with the details of your theme, you need to create an initial comment in the style.css file like so (I called my theme Raul just because…):
Theme Name: Raul
Theme URI: http://www.dezco.co.uk/
Description: A nice clean news looking design.
Author: Robert Desmond
Author URI: http://www.dezco.co.uk/
Tags: responsive, white
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
To add a screenshot of how you want your theme to look you simply need to add a file called screenshot.png to your theme directory (dimensions 600×450). (I took a picture of my drawing on paper until I would finally get a real screenshot!)
There you go! You now have a theme that you can activate although you are probably a fair way from making it look good!