Make a copy for your template structure of /includes/templates/template_default/common/html_header.php
Insert your code just before the </head> tag
Code:
// DEBUG: echo '<!-- I SEE cat: ' . $current_category_id . ' || vs cpath: ' . $cPath . ' || page: ' . $current_page . ' || template: ' . $current_template . ' || main = ' . ($this_is_home_page ? 'YES' : 'NO') . ' -->';


?>

Insert your code here
</head>
<?php // NOTE: Blank line following is intended: ?>