You have to identify your custom template with a includes/templates/YOUR_TEMPLATE/template_info.php file and select it from the pulldown in the admin. As RescoCCC said, Zen Cart will then check the folders for your custom template for the file it needs. If it doesn't find it, it will look in the folders that come with it.
This list is a bit outdated, but it'll give you an idea where to start to create a very basic custom template (you can build from there):
- includes/languages/YOUR_TEMPLATE/english.php
- includes/languages/english/YOUR_TEMPLATE/index.php
- includes/languages/english/YOUR_TEMPLATE/meta_tags.php
- includes/languages/english/YOUR_TEMPLATE/header.php (to change the logo and header text)
- includes/templates/YOUR_TEMPLATE/template_info.php
- includes/templates/YOUR_TEMPLATE/common/tpl_header.php <- may not be necessary
- includes/templates/YOUR_TEMPLATE/common/tpl_footer.php <- may not be necessary
- includes/templates/YOUR_TEMPLATE/images/logo.gif (or .jpg or .png)
- includes/templates/YOUR_TEMPLATE/css/stylesheet.css
Anywhere you see a /classic/ folder, you can create a folder for your custom template. You can copy the necessary files from either the /classic/ folder, the /template_default/ folder or the parent folder.
Edit includes/templates/YOUR_TEMPLATE/template_info.php to give your template identifying information so you'll recognize it in the admin under tools->template selection.
Note: if you've changed the height of the logo in header.php, you may need to change it in your stylesheet as well:
#logoWrapper{
background-image: url(../images/header_bg.jpg);
background-repeat: repeat-x;
background-color: #ffffff;
height:75px; <-adjust this line if necessary
}