1. In the admin panel of your site, open the "Online Store / Themes " section (your_domain/admin/themes)

2. Here you need to choose to edit the code: Actions->Edit Code

 

3. Now in the window you need to find the appropriate template file that contains the closed tag: BODY


In this case, it is the theme.liquid file.

 

4. You need to go to edit this template by clicking on it. In the HTML code, you should scroll down and find the closed tag : BODY.
Immediately before it, you need to add the previously copied Looksize script.

5. Save your changes