Clip the flight at the point where the feather barbules begin on the shaft of the flight feather. There should be no barbules on the shaft of the cut flight feather. Watch carefully for blood feathers. Fortunately these are very obvious. A blood feather will be encased in a spongy sheath and will have black, brown or red coloring in the sheath.
Mature sheaths are hollow and quite hard.
Greys need to be custom cut. Each bird is different in strength and determination. I suggest that four feathers be clipped initially. Watch the bird to see how far he can fly or flutter with four flights clipped. If he is able to fly or flutter more than 15 feet, clip one more feather from each wing.