We have documented the whole process for our theme Photofolio in our wiki (http://wiki.morethanthemes.com/index.php?title=Photofolio:Custom_D6_Installation) and also created a video (http://vimeo.com/21671764) demonstrating the required steps.
In short, the basic steps are:
- Installation of the theme files. Just copy the theme folder (for example [businessclass]) into the sites/all/themes folder of your installation.
- Installation of the theme required modules. Just copy the sites/all/modules folder of our installation into the corresponding folder of your installation. Finally enable those modules through the Drupal administration interface.
- Installation of the theme required libraries. Just copy the sites/all/libraries folder of our installation into the corresponding folder of your installation.
- Adjustments of Image styles through Home » Administration » Configuration » Media » Image Styles.
- Reproduction of all new Content Types. They have to be created by hand or using one of the available 3rd party tools  . We strongly recommend the manual way.
- Exporting of Views using a temporary installation of the premium theme. Importing of Views back to your Drupal site.
- Configuration of the existing blocks to make use of the new regions the premium theme provides.
If you choose to execute the procedure on your end, you can count on us for monitoring and providing you with feedback and tips at no extra cost.
Alternatively, If you do not want or do not have the time to mess with coding in order to carry out the above yourself or within your organization, you can always hire us so to implement this for you. To go for it, please send an email at firstname.lastname@example.org including a link to your website and any information/details on any customizations or implementation you’ve done that you feel we should take into account. Before proceeding, we will check the website, discuss with you any details that might need to be examined, and will inform you about the time needed and the cost of the upgrade by providing you with a detailed Quote. Once agreed upon, we will proceed accordingly.