I will skip designing (drawing, coloring, etc) and image slicing part in design tools such as Photoshop or Fireworks. The most important is testing and customization of Website Interface. This part can be done on few ways.

Custom design extension

You can override default Website Interface design by using custom design extension. Once you have eZ Publish 3.9.0 with Website Interface installed you need to do following:

1. In extension directory of eZ Publish create following directory structure:

   ezwebin_custom
              | - design
              |         | - ezwebin
              |                 | - images
              |                 | - stylesheets
              | - settings

2. In extension/ezwebin_custom/settings directory create new file design.ini.append.php and put inside:

   <?php /*
   [ExtensionSettings]
   DesignExtensions[]=ezwebin_custom
   */ ?>

3. Edit settings/override/site.ini.append.php file and add in block [ExtensionSettings]:

   ActiveExtensions[]=ezwebin_custom
   Note that ezwebin_custom must be placed AFTER ezwebin to override it. For example like:

   [ExtensionSettings]
   ActiveExtensions[]
   ActiveExtensions[]=ezwebin
   ActiveExtensions[]=ezdhtml
   ActiveExtensions[]=ezodf
   ActiveExtensions[]=ezwebin_custom

4. Copy stylesheets from 'extension/ezwebin/design/ezwebin/stylesheets' to your custom design extension 'extension/ezwebin_custom/design/ezwebin/stylesheets'. Note that this will override CSS files only, not template files
5. Clear all the cache, all!

Since now you can start with customization. This is clean and correct way if you are working on new project. Using this method you can override original ezwebin CSS files.

Sitestyle package

This is second method of doing Website Interface design customization. How to make a sitestyle package you can find in official Website Interface customization manual. After installation of Website Interface, two CSS files (site-colors.css, classes-colors.css) are included in pagelayout. You can find them on file system in 'var/storage/packages/ez_systems/ezwebin_design/files/default/file'. Add your CSS styles in those files. Remember that when you specify the path to images in the CSS files, use the following directory nomenclature: images/myimage.png. Just create 'images' directory under 'var/storage/packages/ez_systems/ezwebin_design/files/default/file' then place images used in CSS files there. Those CSS files and images you can upload later via sitestyle package wizard in eZ Publish (see customization manual for more information). New styles will appear immediately after page refresh in web browser.

Still useful might be B?rd's article about customization eZ Publish design with CSS and sitestyle packages.