Log in to your Shopify account, then select Online Store from the sidebar on the left hand side.
From the Themes page, click on the Customize theme button for your selected theme.
At the top left hand side of the page, click the More actions dropdown, and select Edit code.
In the layout folder you should see a theme.liquid file. Select it, then scroll down until you find the
</body> tag. Paste your embed code just before it, as seen in the screenshot below.
You can find your embed code by logging into your Natterly account, and creating a new site.
Verify everything has been set up correctly. In order for the chat box to appear, you need to have at least one user Online, and in a department associated with the site.