Hi there @khitab,

Thanks for posting in the Community! If you are looking to add a header and footer into your Shopify store, I would first look into their documentation for how to get that set up. Afterwards, to learn how to link a Shopify signup form from the footer/header of your website, I would suggest reviewing our instructions found in the Collect Email Subscribers subsection of the Guide to Integrating with Shopify Help Center article. Essentially, you’ll want to ensure you have the Collect email subscribers settings enabled as part of your Shopify integration settings and the hidden input group tag of <input type="hidden" name="contactmtags]" value="newsletter"> installed as part of your Shopify form. 

In addition, here are some resources and relevant information within the Community for troubleshooting further:

Hope this helped!
Alex