One script. Paste it before </body> and the chat button appears on your site.
If your site is served from the same server as this chat (e.g. both on localhost:3000):
<script src="http://localhost:3000/widget/embed.js"></script>
If your website is on another domain (e.g. your WordPress or shop), set the chat server URL first:
<script>
window.ZRK_CHAT_WIDGET_URL = 'http://localhost:3000/widget';
</script>
<script src="http://localhost:3000/widget/embed.js"></script>
Replace http://localhost:3000 with your real chat server URL (e.g. https://chat.yourdomain.com) when you deploy.