Theming
Theming is the work necessary to take total control of the output/layout coming from drupal, without using one of the pre-made themes alone. A new theme can use another theme as a base. It would probably be best to review the basic structure before starting, and then take a look at one or more themes located in the core drupal install.
create the folder
create a folder to contain your theme here: <drupal install>/sites/<sitename such as default>/themes/<your theme name>
create a .info file
.tpl.php files
Create a page.tpl.php file to style the main layout of your site. Then create a node.tpl.php file to control the content output coming from your nodes. If you create a custom content type, you can then control it by creating a node-<content type>.tpl.php file.
create a style.css file
This will control the visual layout of your site.
enable the theme
Once your theme is roughly set up, you need to enable it in the drupal admin, under Administer→Site Building→Themes.
clear-block
Drupal has a build in css fix for when you need a containing element to always extend to the size of the floated children. To use this, simply give the container a class of clear-block. This avoids the common fixes such as placing a div after the floated elements with “clear: both;”.
clearing the cache
Depending on the caching settings of your site, you many not see theming changes right after you make them. To fix this, you need to clear the cache at Administer→Site Configuration→Clear Cached Data
I didn't install it, but there is supposed to be a Dev module that provides an easy link or something to do this faster.