The installation instructions for all the SDO components are in the SDO install section of the SDO documentation.
In any case, the essential facts are that the Relational DAS is written in PHP and it should be placed somewhere on the PHP include_path.
Your application will of course need to include the Relational DAS with a statement like this:
<?php
require_once 'SDO/DAS/Relational.php';
?>