Question about PSD to WordPress conversion-based services

I know there's a few members on here that offer this service but I'd like to know what's used for the coding.

Do they use a basic wordpress theme (such as Twenty Thirteen) and edit the index, header, footer, etc...? or are they creating an entirely new wordpress theme?

PSD > HTML makes sense because it's just coding but I'm confused about the wordpress functionality.

thanks!

add me on skype can help u!
actually didnt get u properly!

you can download more templates for wordpress theme to customize your wordpress site....

This is what I do.

1. I code the theme in plain html. Try to separate the design in structures like header, footer, navigation bar, content and sidebar. Plus code the categories view version of the site.
2. Pick one of the default themes from the last version of wordpress.
3. Remove the original style.css and images
4. Remove all the html code from the required files, such as header.php, index.php, etc, but leave the header tags and php code such as those that call the widgets from the sidebar.
5. Start adapting your code to the theme.
6. Check function.php for any unncessary code.

From there it's pretty straight forward. You split the sections in the correct files and you should be good to go.

When I need to cut psd into wordpress I am getting existing theme and edit it, good point to do so, that you can use for example eCommerce theme or theme with lot of fonts and short codes and page templates.