Took me a while to figure out a way to do them, too. There was a post here on the Nation that showed a way to do them on a lat-pulldown machine. You kneel on the seat of the machine facing away from the tower, with your heels hooked under the knee brace. Then you can grab a triceps rope attached to the cable, pull it down so the ball ends are in front of your chest, with the clip for the rope behind your neck.
Then you can use the weight stack to help, if you can’t do them all on your own. Kinda sucks having your knees that close together, though. You could always just pad out a board to hook to the seat crosswise, to get your knees further apart.
The other way I found that was pretty cool was setting an adjustable incline situp board right up against a high pulley, with the foot braces up against the stack. face away from the stack, hook your heels under the knee braces, and use a triceps rope the same as on the pulldown machine like I said above.
Our situp board was one that adjusted from pretty flat up to at least forty-five degrees. All that adjustment range was pretty cool, you could do GHR’s with your feet way up above your knees, and start at the bottom with your body pointing down at a good angle. I always thought that the dedicated GHR machines should adjust in that fashion.
I hope that all made some sense, there's not really any way I can post pics.