Example
$service = new NetSuiteService();
$search = new TransactionSearchAdvanced();
$internalId = '123';//transaction internalId
$search->criteria->basic->internalIdNumber->searchValue = $internalId;
$search->criteria->basic->internalIdNumber->operator = "equalTo";
$field = new SearchColumnSelectCustomField();
$field->scriptId = 'custbody_os_freight_company';//this is specific to you & found in netsuite
$search->columns->basic->customFieldList->customField[] = $field;
$field = new SearchColumnStringCustomField();
$field->scriptId = 'custbody_os_warehouse_instructions';//this is specific to you & found in netsuite
$search->columns->basic->customFieldList->customField[] = $field;
//and so on, you can keep adding to the customField array the custom fields you want
$request = new SearchRequest();
$request->searchRecord = $search;
$searchResponse = $service->search($request);
