A cutter should be thrown with exactly the same arm speed as normal fastballs but with pressure on the outer half/third of the ball, creating a late cutting motion. You want the hitter to see fastball out of the hand, with the speed and spin, so that he commits a tad early and causing him to miss the sweet spot on the bat.
How to hold a cut fastball:
These pitching grips are extremely similar to normal fastballs. Middle and Index fingers have mostly no space in-between them and they are placed outside the center of the ball, respective to left-handers and right-handers.
There is no real change in the motion of the hand but there can be a slight supination (turning the hand in towards the head). This will help to make sure that when the ball is thrown the force is applied to the outer part of the baseball. In essence, that will naturally cause the abnormal spin on the outer half creating the desired movement away from the pitching arm side.
Cut fastball movement:
The desired movement on this pitch is very slight. You want to maintain close to normal velocity and to do that, the ball can not move more than 3-6 inches. You shouldn't want more movement than that. Remember, the cut fastball isn't designed to be a breaking pitch (i.e. curveball or slider)
How to grip a cut fastball:
Place the middle and index fingers to the outer third of the ball, with no space between them. As far as finger placement on the seams, one possibility is just moving the fingers to the outside part of the seams you use for your 4 seam fastball grip (picture on the left). Another possibility is to place the middle/index fingers in line with the seams like you would a 2 seam fastball but higher up (see Picture on right).
Here are some pictures of cut fastball grips...
4-seam cut fastball grip
2-seam cut fastball grip
How to pitch a cut fastball:
Upon release, try to let the ball feel like it's coming just off the fingertips of the index and middle fingers. Do not attempt to create more spin by letting the thumb take over. Trying to snap with the thumb adds more stress to the elbow. The movement on this pitch is created by applying force on the outer half/third of the ball, as opposed to right through the middle like a 4 or 2 seam. That is where your movement will predominantly come from.
Having your fingers on a certain seam a certain way is up to you. It helps with your ability to feel grip and control over the ball. The only difference is what kind of spin that particular grip creates on the ball and what that spin might look like to the hitter. Again...remember, let the off-center pressure on the ball create the movement and do not try and snap your wrist.
Out of all the offspeed pitching grips, this one could be the easiest to learn at the right time because it's so much like the regular fastball. Remember to let the grip do the work and don't try to make it happen with the elbow.