If you know how to do responsive design already, then just the basics: Each page needs the view port. The css needs to have the media queries. If you don't know that part, then the upgrade will be too much of a stretch. But if that part is familiar, then just address each page in the cart and check out to change basics from pixels to percentages. It's not perfect results, but will get by for now. Personally, I dumped the default index on the cart and made it my own. Skipped all of that part. Then I focused on products results, details, cart, checkout, user login and confirm. Those were the only pages I needed to convert from pixels/tables to percentages. I was using the PP express also. The site I converted is: http://smart-candy-shopper.com
I hope this helps. It's just a matter of looking at the code, finding what isn't responsive, i.e., is still in fixed width in pixels, and changing css to percentages.