After you obtained your keys from Google Recaptcha page, you will need to add this script
in the head section:
<script src='https://www.google.com/recaptcha/api.js'></script>
After that, all you need to do is to initialize the recaptcha by adding the next element into your form:
<div class="g-recaptcha mt-1" data-sitekey="your_site_key"></div>