Copy the embed snippet from the Widget page in your dashboard and paste it just before the closing </body> tag on your site. It looks like this:
<script src="https://app.helpshelf.com/widget/YOUR_SITE_HASH.js" data-helpshelf-site="YOUR_SITE_HASH" async></script>
The launcher appears in the corner of your site within a second or two. Full instructions, including Google Tag Manager and common platforms, are in Install the widget.