1) Get any clear case. Tinted clear cases work but this is just for preference.
2) You can either print out designs within the dimensions of your phone or you can make your own. If one is using traditional media take the paper you want to start with and trace the phone itself. *Note* don't do this with the case on.
3) Cut out your designs and place it into the case. Now, trace the opening in the case for the camera.
4) Remove the design and cut out the traced opening.
5) Slip the design onto the phone and put the case over everything!
6) Repeat for alternative designs. :)