Magento 1.9 Custom Onepage Checkout module
Base module to modify the onepage checkout fields/validation/whatever. This module already includes brazilian taxvat validation. You can disabled it in app/code/local/Wdarking/OPCheckout/Model/Observer.php.
You need to copy the contents of the module inside your magento installation. If you use any custom magento theme, please copy the contents of the app/design/frontend/base/default/template/wdarking/opcheckout to app/design/frontend/<your-theme>/default/template/wdarking/opcheckout.
You also need to override the onepage checkout template files, and add the wanted custom blocks in the templates.
Example: custom company fields in onepage checkout billing page
You should copy the file at app/design/frontend/base/default/template/persistent/checkout/onepage/billing.phtml to app/design/frontend/<your-theme>/custom/template/persistent/checkout/onepage/billing.phtml and insert the following line in the file:
<?php echo $this->getChildHtml('wdarking.opcheckout.companyfields'); ?>