Easy procedure...
Hi there...
Its quite easy to use the Hertz ZE 1 award certificates. Last I used them, it was necessary to get paper certificates. (I don't think this has changed, but I haven't been following it closely).
Call Hertz #1 Awards 800 number and request the appropriate certificate... They'll give you the PC # corresponding to the certificate. Use this PC# and make the reservation. (Check the black-out dates on hertz.com first - these are never really a problem for me...)
Good luck...
-R