Each of the files within this Side by Side checkout page tutorial folder you downloaded should be placed in your code snippet plugin and activated.
Some of the files are CSS and others are PHP, make sure to properly assign each to the proper catagory.

You may need to add an additional page template to achieve a proper page layout: For this checkout page we found that the FW No Sidebar page template was suitable to achieve the desired layout. PLUGIN utilized is:  Fullwidth Page Templates by https://brainstormforce.com/

Every website has different alignment and styling, menu cart code will need some adjusting to your sites styling and alignment. The video gives you information on how to easily achieve this. 