See README file of the Web service client module.

This is a sample way to call a WSClient operation:

  $service = wsclient_service_load('your ws service');
  
  $account = new stdClass;
  $account->name ='your user';
  $account->pass_raw ='your pass';
  
  // For own HTTPS certificates.
  $curl_options[CURLOPT_SSL_VERIFYPEER] = FALSE;
  // For own HTTP authorization.
  $curl_options[CURLOPT_HTTPAUTH] = CURLAUTH_BASIC;
  $curl_options[CURLOPT_USERPWD] = $account->name . ':' . $account->pass_raw;
  $service->settings['curl options'] = $curl_options;
  
  try {
    $result = $service->your_method();
  }
  catch (WSClientException $e) {
    watchdog('wsclient', $e->__toString());
    return null;
  }

  return $result;

This next example assumes that you have a POST method configured in a REST service. Use a parameter on the method as a placeholder to store your data.

Passing arguments to your methods is also a fairly straightforward process, in the following example a POST request is being made with a REST WSClient. Some data is being passed as the payload:

// Function that takes a $node to make a POST call with.
function example_post_call($node) {
  $service = wsclient_service_load('service_name');
  // An array that will be serialized into JSON or XML depending on your configuration.
  // Various settings in you service's "request formatter" determine how this array is
  // transmitted. Ensure that there is a parameter set in your method where this can be
  // stored.
  $data = array(
    'name' => $node->title,
  );

  try {
    // The method called here is the machine name of your operation.
    // You can also call invoke('my_post_method', $data) instead.
    // We pass $data in here, where it will be used as $arguments for this WSClient, 
    // which in turn will be used to create the 'parameters' and 'data' for the request.
    // If you have more parameters required for your service, those need to be passed
    // here too.
    $response = $service->my_post_method($data);
  }
  catch (WSClientException $exception) {
    watchdog('example', $exception->__toString(), array(), WATCHDOG_ERROR);
    return FALSE;
  }

  return $response;
}

/**
 * Implements hook_wsclient_rest_request_alter().
 */
function example_wsclient_rest_request_alter($request, $service) {
  // This hook can be used to modify anything in the $request object just before it is 
  // handed off to CURL. In this example, we want to unset the data from the
  // parameters, and an api key from the data. This is because the REST client takes all
  // the arguments and parameters and copies them into the data element.
  unset($request->parameters['RequestRootName']);
  unset($request->data['key']);
}

It should be noted, again, that anything passed to your REST service method ($data in the above example) is inserted into both the parameters and data of the request.