"The template overrides must be defined in the "override.ini.append.php" file of a siteaccess".

This means that the file should be created in the /siteaccess/your_site/ folder, next to the site.ini.append.php file, which resides in the same folder.
So, for every site access, you have one override.ini.append.php file.

The templates that you create to override the standard templates, are placed in the /design/your_design/override/templates folder (create it if nescessary).

Custom pagelayout templates however, can just be placed in the design/your_design/templates folder. There is no need to create a override.ini.append.php for those. If templates exist in the /design/your_design/templates folder, they will always be used instead of the /design/standard/templates folder.
See also: <url>http://ez.no/doc/ez_publish/techn...igns/design_combinations</url>

The special thing with override templates is that they can *conditionally* override default templates, only if cetain conditions are met, like node id or object class, using the Match specifier.