I'm trying to install this on two legacy sites (both zc 1.3.9h) for a client. Will Google reCAPTCHA for Zen Cart v3.3 work on a site that old?

I am able to complete Steps 1 & 2, but I'm confused by Step 3 under "Instalation for Zen Cart Versions prior to 1.5.1":
...manually add the <?php echo recaptcha_get_html(); ?> to your own template files

Which template files and where do I add this?

Thanks!