I've never seen any Zenith-created "how-to" photo instructions, but the procedure isn't all that difficult and is basically the same for all Robot Dial sets from 1938. The following link provides a verbal walkthrough of the process. It was written for a Robot Dial model equipped with motor-assist tuning, so the section referring to the motor drive belt can be ignored if your set isn't so equipped.
http://www.antiqueradios.com/forums/viewtopic.php?f=10&t=233664