I have some kind of compass type thing I use. I believe its made by Wal-Borard Tools. You tun the knob and set the diameter you want. The spike goes on the midpoint and you turn it to score the sheetrock. Then I use a keyhole saw to cut out the circle along the mark scored by the tool. I've had too many hole saws lose control and go for a quick dance across the ceiling, tearing up the ceiling, so i try to avoid them.