To add the recommendation widget to checkout page, we need to use Editing Theme files (Advanced) approach. 

Note: If you are theme developer, skip this tutorial and use your local stencil development environment to insert the widget HTML snippet . Otherwise, please continue reading. 

1.  Go to BigCommerce Admin Panel -> Storefront -> My Themes 

2.  Under "Current Theme" section, click on Advanced -> Edit Theme Files to open the Stencil Theme Editor in a new window. 

Note: if you are using a out of the box BigCommerce theme, you may not see the "Edit Theme Files" option. You will need to click on "Make a Copy" first to make a copy of your theme, then apply the theme copy as your current theme in order to edit the theme files. See BigCommerce official document for details.

3.  On the left side of the Stencil Theme Editor, click on templates -> pages -> checkout.html to open the checkout page html template. Note: the files layout and content may vary based on different themes. 

4.   In the checkout.html file, add the widget HTML snippet  at the desired place. 

5.  Click on "Save & apply file" to save your changes. It may take several minutes to save and apply the changes to your store website. 

6.  Now go to the checkout page of your website. Voilà, the recommendation widget shows up on the checkout page!