In the past entry forms have been available, for advance entry, from several shops & offices in Hythe - Blue Shield Insurance for example, or from the Race Organiser, Greg at Kent Trophies in Dover.
Lots of people choose to enter on the day at Saltwood Green - expect a queue if you plan to do this.
However, I've not seen any info about this year's run, so I'm not sure if any of my comments above will be relevant.