The Two-Seam Fastball Grip- Grab a baseball and find the area on it where the seams are the closest together. Rotate the baseball so that those seams are perpendicular to your body. Place your index and middle fingers on each of those seams respectively. Each of your fingers should be touching the seam from the pads or tips to almost the ball of each finger. (See photo above- Your thumb should rest underneath the ball in the middle of those two fingers.)
What I ask my pitchers to do is rotate the baseball directly towards their palm so that the thumb will find the apex of the horseshoe part of the seam. The thumb needs to rest on that seam from the side to the middle of its pad.
You are now ready to throw a two-seam fastball. This ball will tend to move for the pitcher a little bit, depending on velocity, arm-slot angle and pressure points of the fingers.
This is not a curve ball or slider. If the ball breaks hard then make sure the pitchers fingers are directly behind or on top of the ball and not on the side at release.