There is no such thing as an assistant in Kohana 3 / 3.1
You create a class and use it, as usual, in a PHP application.
The only requirements are that the classes are in the directory classes, and the underscore in the class name is equal to the directory delimiters. for instance
class HTML_Helper
will be placed in
classes/html/helper.php
Then this is a simple example of using the class, as usual.