Step 1:
From your Shopify admin click Online Store:
Step 2:
In the theme that you want to edit, click Actions and then click on Edit code:
Step 3:
Click Layout and then theme.liquid file:
Step 4:
Add the following code wherever you want the widget to show:
<div class="sellify-ucc-wrapper"></div>
Step 5:
Click save and that's it.