User manuals

All instructions about how to set up Cookie-Script properly

Go to your Cookie-Script.com account click "use" button and copy your item's src attribute's value (i.e. file URL)

opencart

Open catalog/controller/common/header.php and add before

$this->data['scripts'] = $this->document->getScripts();

this code, where your-file-URL is your item's src attribute value. 

$this->document->addScript('your-file-URL');

Note
Also there is possible just to place your script inline into catalog/view/theme/{theme name}/template/common/header.tpl using normal HTML markup.

Cookie-Script is installed on OpenCart now and should be ready for use.