Mariano Rivera holds his cutter grip like a four-seam fastball. However, he moves his fingers about a centimeter to the right on the ball so it is closer to the curved horseshoe part. He applies a little extra pressure on the index finger and puts his index finger slightly out front as he throws.
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Mariano Rivera throws his Cutter, or "Cut Fastball" by simply starting with a two-seam fastball, and moving his Index and Middle Fingers to the right until the seam of the ball is in-between both of the Index and Middle Fingers. He then just throws it. The angle in which you are holding the ball will make the ball automatically brake toward left handed-hitters, and brake away from right-handed hitters. That is if you are a righty. It is the opposite if you are a lefty.