1. Log in to your Shopify admin.
2. On the left side, select Online Store under Sales Channels.
3. Choose Themes.
4. On your Live template, click Actions and select Edit Code.
5. Select your main *.liquid Layout.
6. Add the ProfitWell snippet before the body element and replace PUBLIC_API_TOKEN_HERE with your public API token.

<script id="profitwell-js" data-pw-auth="PUBLIC_API_TOKEN_HERE">
    /* DO NOT DELETE - this code is used to accept our payments or send data to ProfitWell*/
    (function(i,s,o,g,r,a,m){i[o]=i[o]||function(){(i[o].q=i[o].q||[]).push(arguments)};
    a=s.createElement(g);m=s.getElementsByTagName(g)[0];a.async=1;a.src=r+'?auth='+
    s.getElementById(o+'-js').getAttribute('data-pw-auth');m.parentNode.insertBefore(a,m);
    })(window,document,'profitwell','script','https://public.profitwell.com/js/profitwell.js');
  {% if customer %}
    profitwell('start', { 'user_id': '{{customer.id}}' }); 
  {% else %}
    profitwell('start', {});                     
  {% endif %}                         
</script>