Go to graflex.org and look around for more information. The lens takes the common Series VI filter - you unscrew the built in lens shade, drop in the filter and screw it back on. There are hints on the graflex site for getting a stuck lens shade off.
I'd caution you to be very careful cocking the shutter. There are plastic/nylon parts inside and the only man who still knows how to repair them had a stroke last spring.