VTiger: WebService - How to Create a custom webservice method that is exposed by REST interface

Question

I want to create a custom function and then invoke it using REST APIs. It's not clear how to do that exactly on vtwsclib documentation.

For example i need to know how to create custom_wsmethod() to use it on the following example:

<?php
include_once('vtwsclib/Vtiger/WSClient.php');
$url = 'http://en.vtiger.com/wip';
$client = new Vtiger_WSClient($url);
$login = $client->doLogin('admin', 'KpS9EjNz16JtPmoe');
if(!$login) echo 'Login Failed';
else {
 $response = $client->doInvoke(
 'custom_wsmethod',
 array( 'firstparam' => 'F', 'secondparam' => 'S',
 'POST')
 );
 $wasError = $client->lastError();
 if($wasError) {
 var_dump($client->lastError());
 } else {
 // Take action on $response
 }
}
?>