the critical component of any change up grip is getting the fingerprint pads of the index and middle fingers off of the baseball. you can do that many ways. the conservative method is by moving the index finger off the ball and the ring finger side of the middle finger is the only part of the middle finger that rides on the ball. most people call this the circle change and emphasize making the circle with the index finger. i have better luck emphasizing getting the pad of the middle finger off the ball.
the next method that i have good luck with is emphasizing the position of the ring finger on the ball. if the ring finger and thumb are on the middle or equator of the ball and you tell a kid to throw a fastball off of his ring finger, that tends to work very well. they do not slow their arm down and can develop feel with the pad of the ring finger. (this is pedro's change up)
the aggressive method is to throw a two seam fastball off the ends of the ring finger and pinkie. don't even think of doing this unless you are prepared to work. you will need to throw this pitch at least 50 times per day to develop feel with it. it is a quality change up.
the most radical change up grip i've ever seen (only used by one pitcher i know) uses the two seam ring finger/pinkie grip parallel to the seams, with the fingers split apart outside the seams (a type of split finger using the ring finger and pinkie). it is tough to master this pitch, but if you can throw it for a strike, it is devistating. you do not have to turn it over or anything. rotate it straight backwards so it looks like a fastball coming out of your hand. it will never get there.
this thing is devastating if you can do it.