You will need to setup your CNAMES with your DNS hosting company before following these steps.
Login to your Emercury panel and update your Branded URL's. To do this go to "Account Settings - Branded URL's" located at the top right of your panel.
Enter your img.yourdomainname in the field that says Image domain. Click Check Domain.
Enter your url.yourdomainname in the field that says Click domain. Click Check Domain.
Enter your track.yourdomainname in the field that says Track domain. Click Check Domain.
You will see a green check mark if everything was set up correctly.
Thats it! You're done. That wasn't so hard was it? Let us know if you have any questions.