You must have a Google Analytics account to complete this process. To sign up, visit analytics.google.com. Twitter will use this to make your timeline better.
Once this is on your toolbar, you'll be able to make a TinyURL at the click of a button. With TinyURL, you can also make a smaller URL that will work for any page on your site.
To verify that the tracking pixel is firing, make an order for the event you added the tracking pixel to. See if you are using Classic Analytics (ga.js) or Universal Analytics (analytics.js).